Baritone/Tenor Horn/Euphonium
Use a floor stand and have the lead pipe (attached to the mouth piece) moved to the other side of the instrument.
Cello
Use an Artiphon Instrument 1 in ‘Tap’ mode to emulate the sound and tuning of these and other instruments. Or a bow holder can be attached to the wrist.
Electric Guitar/Bass Guitar
Use a Chapman Stick one-handed to create an electric guitar/bass guitar sound using a ‘hammer on‘ technique. Or attach a plectrum like Tony Memmel.
Piano
The music exam board ABRSM now has a syllabus for Performance Grades 1 to 5 for anyone playing the piano with one hand.