Thank you, Hauke D.

I ended up rethinking that part altogether and moving method implementation (i.e. the do_foo()) part into a separate class (aka strategy pattern). Should I really want to provide in-place callbacks, I can always write an in-place package with only needed methods overridden. Actually already did it for the test.

I was doing it wrong. At least a bad decision was caught before it became hard to remove.