(Though the tonal music Schoenberg wrote after that, memory serves, wasn't much centered in C major, but in G major (suite for strings), D minor (organ variations), E-flat minor (completion of his 2nd chamber symphony), G minor (wind band work), etc. Just saying.

Hasn't Boulez been conducting Romantic music for quite awhile? Mid or late 1990s certainly- I remember reviews in Fanfare of recordings that also mentioned broadcasts - but maybe well before that too for all I know... (of course, the works most often mentioned- Bruckner and Mahler for instance - are not only Romantic but pivotal to the history of the "Modern tradition"- and not just incidentally so but musically too, I say- and Debussy's Prelude, I agree with Austin, is modern despite being pre-20th century...)
As to deservedly unsung... for starters, the vast majority of dissertation compositions (in any style, "modernist", "eclectic", "neoRomantic" or otherwise.) It's a rare graduation-work that, like Prokofiev's 1st piano concerto (or Myaskovsky's 1st symphony, or etc. ...) deserves to be heard again much, far as I know...