Catching unescaped pipes

Yes, {{rfv-failed|<span class=foo>quux</span>xyz|bar}} would also fail to be caught (it should be noticeable visually, though). I fear some performance problems with that solution, though. Maybe add it to ugly hacks for now.

I added a check_for_extra_args function to Module:ugly hacks (I haven't tested it yet). I don't see why the performance would be bad at all (the slowest part would probably just be loading the module), especially since these archive templates tend not to be transcluded too many times on one page.

In this instance probably not. However, I think it would not be necessarily desirable to use it very widely either. I think it defeats lazy template argument expansion, for one.

Maybe one could ask at bugzilla: for some "unused arguments tracking" functionality for use in modules.

Well the only purpose I really intended it for was the archive templates, so I agree it shouldn't be used anywhere else.