Alright so here's my thoughts, I think nearly all 3rd or 4th generation 20th c. composers who aren't Philip Glass, Stockhausen, etc. are going to be "unknown" because nobody mainstream listens to art music from that period anyway.
Kapustin's popular with piano geeks and Japanese because it sounds anime and jazz fusion. Vine, Koechlin, etc. are more striking because they're romantic composers and when we think of piano music, we think of romantic music, so obviously someone from the romantic period who isn't known is SOMEONE.
Moszkowski is too well-known already from either the typical etudes Op. 72 teachers keep assigning to students or the Concerto which is again, like Kapustin, known to piano geeks.
So, my vote goes to Theodor Dohler:
This guy's etudes deserve to get an album recording imo.