Okay, that was not the impression I got from the implementer I was consulting with, but surely there's someplace we can put a conformance table. Can we not store a pointer to it wherever @Douglas_Gregor was talking about putting his decision procedure ("in some runtime hooks we have during witness table realization")?
It seems to me that if there is absolutely no ABI place to tuck additional information into generic function call, there is no hope for a sane resolution of conflicting conformances in multiple modules without breaking the ABI.