Attached is a proof of concept for the blacklist approach (for 3.4, but without the fixes needed for the transform codec handling tests in test_codecs)

This does have the potential to add a reasonable amount of additional overhead to encoding and decoding for shortstrings. Since it isn't obvious where to store a set for faster checking against the blacklist, it may be worth benchmarking this naive approach before doing something more complicated.

Regardless, I don't plan to take this further myself any time soon - I just wanted to give it a firm nudge in the direction of the blacklist approach by providing a proof of concept.