Well, sort of...
On the CD of the studio recording of "Lady, Be Good", which is a restoration of a Broadway show from 1924 by George Gershwin, one of the characters, a slimy lawyer, is played by none other than Jason Alexander...I didn't know it until the second or third time I listened to the disk, and I said, "hey, that voice sounds familiar...", and I looked at the cast list, and there he was! He's even got a couple of songs on there...