Here is another piece that can be added to this list:
Gilles Tremblay's Fleuves.
Though written in 1976, probably for the Montreal Summer Olympics, it only has been played in concert twice. Once in 1977 by the Montreal Symphony Orchestra and the other representation is being given right now!
The Réseau Orchestra (an orchestra formed with the best musicians within the 7 music Conservatories in Québec) is on tour playing this piece, along with Debussy's La Mer and Mahler's Lied von der Erde.
Concerts are/were/will be performed in Montreal on May 27th, in Val d'Or on May 28th and in Québec City on May 30th.
The part specifically calls for "Trombone contre-basse". It features plenty of low notes, including some exposed pedal E's, F#'s and G's. At the very end of the piece, there is a "game" being played by both the contrabass trombone and the tuba, where they exchanges pedal F#'s with only the percussions reacting to them (us)
Sadly enough, I don't have the instrument to play this piece, so I play it on a regular Bb bass trombone. It is sad, but I live with it.
Gilles Tremblay's bio can be found here