Point 3 means that if I have to go and modify toke.c, perly.y, and various bits of gv.c (for example) to make this work reasonably well, thus making it work only for versions of perl with the mods (likely 5.8.1 and up) that's OK. I'd rather not have to, if for no other reason than to avoid messing around in toke.c, but...
Point 2 means that the pragma must be lexically scoped. It should only complain about uses of undefined subs in the blocks that it is in force for, so it doesn't complain about modules and whatnot that might otherwise trigger the warnings. Much of the perl library uses subs that only exist on particular platforms, with $^O conditional tests to make sure they don't get tripped over otherwise. We don't want to yell about those, nor modify the standard modules to be completely sub-safe.