Okay, so what I've been doing lately at least a couple of people saw at YAPC:
The tricky bit here in both cases is doing it without touching the base code at all. If you're interested in the gory details, see Devel::Command and WWW::Mechanize::Pluggable on CPAN.
I'm currently looking at the possibility of extracting all the special-purpose code in WWW::Mechanize::Pluggable into a pair of base classes - Class::Pluggability and Class::Pluggability::Plugbase, so anybody can write a pluggable wrapper for any module whatsoever.
And this is actually part of my actual job - woo! I'm having fun.