Well, I would call the following a list of "Smelly Cheese" composers.
These are basically 20th century composers influenced in one way or
another by "the emancipation of the dissonance" and/or microtonalism,
polytonality, etc. I found that after I started listening to this type
of music, I was spoiled -- I couldn't go back to eating Velveeta and
Mild Cheddar, or to listening to bombastic 19th Century composers
hurlling major triads at me. The problem is that these composers will
not make you very popular with your neighbors, girlfriends, coworkers,
etc. If you listen to Robert Schuman you will hate these composers. If
you listen to Cecil Taylor and Ornette Coleman (on the other hand), or
maybe to Captain Beefheart, you will know exactly what's going on.
Smell Cheese. Fine Brie. A Liederkranz of music.
My main nominee at the moment would be Vartein Valen, the greatest Norwegian composer, one of the last great symphonists, composer of some of the greatest 20th century piano works after Schoenberg, and absolutely exquisite orchestral songs. I have decided that he was a genuine genius. I think that Alan Pettersson is now pretty well covered and recognized, and now needs performances more than he needs recordings. The great leader of 20th century Scandinavian composers, Hilding Rosenberg, is in fairly good shape via nice recordings of his quartets, etc. I think that there are some key works of Karl-Birger Blomdahl remaining to be recorded. I guess that's about it for Scandinavia.
There's a wonderful generation of American composers influenced by Schoenberg who are in danger of being completely lost in favor of minimalist sludge: My primary candidate here would be Wallingford Riegger, but there were many other interesting composers from the twenties through the sixties, including Alvin Etler, Ross Lee Finney, etc. who combine (to greater and lesser extents) the "emancipation of the dissonance" and in some cases serial techniques with a genuine "American" feeling (Riegger's Fourth Symphony is a great example of this). To repeat, this is a an area (out of national pride also, by the way) that deserves more exploration. (A footnote belongs here to look into Schoenberg's lesser known pupils, both European and American -- Hoffman, Adolph Weiss, etc. -- who were sparesely represented on recordings during the LP era, but who have now disappeared totally from sight).
Of the Brits I'm a member of the Brian Society, and you've already got him on your list. Humphrey Searle and Still (the Brit, not the African American composer) are in the process of being completely forgotten. Searle was very much influenced by Alban Berg, rather than by typical British pictorialism. I know two of his symphonies via old, long out-of-print Lyrita LPs. Frankel seems promising on the basis of several recent CPO releases. Alan Rawsthorne was a mixed bag: His piano concertos (recently released on CD) are kind of typical world-be crowd-pleasers that have a sort of dated forties/fifties feel, but his quarters (once available on LP) and his solo piano works are another matter entirely -- truly wonderful, deeply felt chamber music. I think that there's more to be discovered here. You'd never believe from the couple of Hispanic schtick pieces that have occasionally turned up on recordings, but Roberto Gerhard was a serious composer, whose chamber music in particular deserves attention.
As a footnote to the Brits, Priaulx Rainier was a very interesting (if I recally correctly) South African woman composer who had one or two pieces on disc in the LP era. She fits in very nicely with this group, I should say.
Farther afield, Julian Carillo, the Mexican microtonalist, was a wonderful composer: Only one of his works (the MASS FOR POPE JOHN XXIII) was ever available on a U.S. label(CRI), but many works were available at one time on Mexican LP's: PRELUDE TO COLUMBUS, HORIZONTES, etc. These would get a great response among the "New Age" crowd, and have the virtue of not being maddeningly repetitious (like most of that type of music). Carillo'w felow microtonalist Alois Haba is finally getting a plethora of recordings -- in the LP era there was one Supraphone disc of quartets and that was that. If you like the other composers on my list, you'll like him, I think.
Of the German/Austrian school, thanks to CD's most of the composers are decent shape, but more needs to be done for Ernst Krenek (thanks to CPO we now have two symphonies and the quartets -- but lets get more the orchestral works, and that wonderful vocal chamber piece SPATLESSEN). Thanks to the LaSalle Quartet and Paul Zukowsky, Artur Schnabel has had some fring recognition as a composer, but deserves much more -- he was a real original, you can't pigeon-hole him. I think that most of the great works of Zemlimsky are finally pretty well known, thank goodness, and the same goes for Franz Schreker (with posthumous apologies to Ernst Krenek for mentioning him and Schreker in the same paragraph).
In France, more needs to be done for their post-Webernian composers other than Boulez: For example, the piano sonata of Jean Barraque is regarded (I believe) as one of the great 20th century piano works. It was formerly available on a recording by Yvonne Lorriod (would someone please get the rights to this and reissue it on CD!) I see that a second cycle of Milhaud (German!) is making its way on the CD shelves. Milhaud is hardly an "unknown" composer, but he falls into a category of serious composers who are now known primarily for silly, commercial works -- in the case of Milhaud, for his Brazilian-flavored stuff. Remember that Milhaud dedicated his second (?) quartet to Schoenberg. (Walter Piston and Roger Sessions were once in this same category, from which they have posthumously liberated themselves.)
Malipiero used to be obscure, and I treasured my 1/4 tapes from the Library of Congress (World Premiere of the wonderful 8th Quartet) and KPFA radio. Now, thanks to Marco Polo and others, he's "out there." He was interesting a lot of the time, but for me the late period (the "CORNAMUSE" symphony and the aformentioned 8th Quartet) stand out, and I would really like to see more of the later chamber music recorded. This is another example of an "older" composer who went into a really interesting final phase under the influence of serial techniques. (Why oh why didn't Brian, an admireer of Schoenberg's, live to age 104 and write 12-tone quartets?)
The CD revolution has created this weird phenomenon in which music that is neither played on the radio nor performed in concert halls is available to buy at Tower Records. This is great -- 25 years ago I ran out of things to buy very quickly. On the other hand, how long will this last (with great record labels such as my favorite, CPO, and Marco Polo) if there is no "marketing," by which I mean no exposure of this music to the public other than via its availability for purchase? This is truly the paradox of the fate of "The Unknown Composer" in the 1990s.