@slimechildaudio – that is awesome. Even in a sadly (though understandably) unreleased module, I am glad at least someone has pulled this off, and I am not surprised that it was you!
@InfrasonicAudio – here’s a (very!) recent code snippet and link to the API–
and here’s an older thread (some parts may be out of date, but a lot should still be valid):
AFAIK there’s no current developer reference, but there are 77 open-source expanders at time of writing, ranging from extremely simple to extremely not. I don’t know if any use a version of the weakref technique Coriander got working.