"%_<filename" has a purpose in the debugger (see %DB::dbline), and that may be what the author of the patch meant by "the %_ hack".
I also noticed that the module "English" assigns the whole typeglob (*_) to the name "ARG", i.e. @ARG becomes equivalent to @_, $ARG equivalent to $_, and therefore %ARG becomes equivalent to our mysterious %_. This further bolsters the theory that it's just a leftover from the others in the same typeglob.
> Since _ variables ($_, @_) hold arguments, would %_ make
Doesn't seem intuitive to me, sorry. I prefer named variables.
> Other than declaring it local (which suggests some deeper
Definitely hinders. In general, you want to stay away from using global special variables in the way that they were not intended, even if they appear as if they are not being "used" at the moment. Future versions of perl may do something with the %_ hash - who knows? The whole reason you cannot declare it with 'my' is because it is a special global variable. You can localize these, but not "my" them. Better to just create something like %subargs, or even %_subargs if you really like the underscore. :)
Finally, as to your lack of editing problem, try complaining at the Editor Requests section. It may be a bug in the code, as sometimes (albeit very rarely) I cannot edit my own nodes either.