Mouse and keyboard dont have to use an amiga side usb stack.
Unfortunately Ive never been able to find out exactly what the story is with amithlon and posiedon. At one point it was supported, but support was withdrawn I believe for some reason or another (driver related maybe).
Id be interested to find out what the story with it is myself. Seeing as my amithlon box is connected to my file server I dont really need usb mass storage, and I was lucky enough to find a joypad that runs off of ps/2 connector (essentially a joypad shaped keyboard, the buttons are actually keys).