I am mostly replying to this so I can get notified as people post - I hope! I play clarinet in a band that has frequent soloists, and finding a piano solo is tough. The literature just isn't there. There is a transcription of Grieg's concerto that is not too bad, and one of Gershwin's Rhapsody in Blue, but everyone would be hearing them (yawn) again. Percy Grainger loved "significant" piano parts in many of his band pieces, such as Gumsuckers and Childrens Marches, but neither of these is really a solo for the piano.
So I will be curious to know if anyone scares something up - I keep fantasizing that Prokofiev wrote something that is somehow obscure!