Yeah, got a variation on rs_usedb to work (assigned to a new function string class). Between switching in/out the new function string class ... and a few manual sp_recompiles (to reset RI constraint violation checking due to using previously cached statements) ... finally got a kludge to work.
The new/custom function string class was a little easier/quicker than setting up a login trigger (that reads the desired disable_ri_check setting from a config table).