OK, after a bit of experimentation, it appears both 3.2 and 3.3 eventually get annoyed if you mess about too much with __pycache__.

1. They're both fine if __pycache__ is entirely unwritable (they just silently skip caching the bytecode)

2. 3.2 throws EOFError if you replace the cache entry with an empty file, 3.3 silently rewrites it with a valid cached version

3. 3.2 throws EOFError if you replace the cache entry with a directory, 3.3 throws a more accurate IsADirectory error

That means my chosen test case is a valid one, and I can just update the offending call in importlib._bootrap to use the new frame stripping hook as I originally planned.