> I assume the hard part will be getting the details for Linux (doesn't bitness 
> have to be compiled in there? For Windows it can be determined at compile-
> time...), but preferring an extension with the ABI tag and falling back on
> one without seems easy enough.

Sticking to bitness should be easy (although I wonder if it would be desirable for platforms with fat binaries - Ned?). If we can go the extra mile and include platform identification all the better, of course.