if a vendor has produced drivers for OS X, does that make it any easier for them to port such beasts to Linux given the BSD underpinnings?

Not really. Apple didn't use the driver model from an existing unix instead creating their own know as IOKit. IANADD (I am not a driver developer ;-) but from what I understand writing OS X driver kexts is significantly different from writing drivers for linux or other BSDs.

-Mike