This WILL be random.
And, I mostly thought of this since these kinds of Composers usually seem to only have one Opera under their belt. The first two that come to mind are:
Ferneyhough 'Shadowtime'
Feldman 'Neither'
Stockhausen 'Licht'
Ligeti 'La Grand Macabre'
Lachenmann 'The Little Matchstick Girl'
Birtwistle
Boulez
Lutoslawski
Penderecki 'The Devils of Loudon'
Penderecki "Paradise Lost'
Webern
Messiaen 'St. Francis of Assisi'
Xenakis
Blomdahl? 'Ariana' (first "space Opera")
PHILIP GLASS (yes, technically High Modernism)
oh... drrrr..... Zimmermann 'Die Soldaten'
Cristóbal Halffter: Don Quijote (available on CD--Glossa), Lázaro (DVD--NEOS), Schachnovelle (not available AFAIK).
Gilbert Amy: Le premier cercle (CD--MFA Radio France)
Luigi Nono: Al gran sole carico d'amore, Intolleranza 1960, Prometeo (?)
Henri Pousseur: Votre Faust
A whole lot of Luciano Berio pieces.
Bruno Maderna: Don Perlimplin, Satyricon, Hyperion (?)
Alexander Goehr, Philippe Manoury, Michaël Levinas...
Alberto Ginastera: Don Rodrigo, Bomarzo, Beatrix Cenci
Luigi Dallapiccola: Volo di notte, Il Prigioniero, Ulisse
Roger Sessions's Montezuma
Elliott Carter: What next? (here the "?" is part of the title ;) ).
Emmanuel Nunes: Das Märchen
Goffredo Petrassi: Il Cordovano
Giacomo Manzoni: Atomtod, Doktor Faust
Late Krenek?