Baritone Treble Clef
Nearly every concert band score includes a part for treble clef baritone/euphonium, this due largely to the overabundance of trumpet players and the shortage of baritone players. Please add that instrument option. The best I can do now it to add another trumpet/cornet part and then transpose it down an octave.
Comments
ISTR that this is possible already, some brass instruments are offered twice, with different clefs and transpositions, check all instruments, not just the common ones, or use the search box in the instruments dialog
It is indeed already possible.