So I consider UNIVERSAL::isa() to be a "best practice" because it gets (as near as I can tell) all but the one rarest case right...

Unfortunately, the function use of isa() and can() has been deprecated in bleadperl for at least a year. It doesn't answer the question appropriately. It's a hack. It relies on an accidental implementation detail of how method storage works in Perl.

In my mind, the right solution is to patch overload to answer DOES() appropriately, which has the benefits of being the right question, giving the right answer, and not breaking methods. (Life would be even more pleasant with autobox in the core at that point; then reftype() would be unnecessary before calling DOES().)