I haven't tested yet (on Windows ATM), but this is exactly what I was thinking to do.
The only question is whether the semantics make any sense, as in whether returning nothing in the Python function should default to returning `FALSE` to Geany. The [API Reference](https://www.geany.org/manual/reference/keybindings_8h.html#afb2861d240a29818...) makes it sound like this is not the normal case.
It may be useful in either case to improve [the doctstrings](https://github.com/geany/geany-plugins/pull/809/files#diff-8c19fc53e5ac66527...) to clarify this, as a separate change if need be.