Highrez

I use an old Logitech MX500 that I will use until it is a small lump of plastic with buttons and a wire.

XBMC is the only thing that keeps it functional with newer versions of Windows and I was wondering if it's possible to return some use to the 'Quick Switch' button.

Originally, clicking it would allow you to effectively Alt-Tab with a small Logitech UI.
I don't need it to act as it did, but currently XBMC does not see that button at all and it has no functionality.

Yeah, it's a feature request, and it's probably been asked before... searching the forum I couldn't find mention of it though.

Sorry, not possible. The FAQ explains why XMBC can only see and remap up to 5 buttons.
The button you refer to does not get sent to/seen by Windows as a normal mouse button (one of the 5 Windows supports natively).

I suspect it would be something like that or it would already be available.