FWIW, Faré's example, where he has to push /both/ the CFFI path and the CFFI-UFFI path into the asdf registry shows the risks of this kind of behavior.
Mightn't it be better to provide an easy SLIME command to modify the search behavior on the fly, than try to mess around with single-file patching? That seems very likely to fail for systems that have nested or related subsystems (UFFI example, related -test system, etc.).
Cheers, r