Well, I got my SL6000 for Christmas, and Hubcapboy has kindly given me an iRiver adaptor while he was over here in the UK.
So, I've taken a look at the USB mouse issue - it'll work, guaranteed
Plugged in my USB mouse, then look at the system info - there it is, listed nicely.
Now, to see if it's actually working I did:
cat /dev/input/mice
This spits out garbage when you move the mouse - just what we want to see
A simple applet to deal with this will be a doddle. For simplicity, what I'll probably do is poach the cursor code from InputHelper and combine it with something that watches "/dev/input/mice" for input in the same way IRK watches /dev/ttySx. Should be able to do it in an evening (maybe tonight if the wife doesn't mind me throwing a late one).
I'm not sure what the various protocols USB mice spew - the first version will probably only support the protocol from the mouse I use (but more can be added later, and the nice thing about USB is that it's possible to auto detect what mouse it is, so there's no need for any sort of configuration).