How difficult would it be to port this to mailman?

i'd be interested in this too...

A good question ... I've used Mailman and I know it's more common than Sympa. The reason this only works with sympa for now was that it was the only mlm that provided this kind of interface (e.g. the ability to create new lists, and to query it directly for membership in lists). The code is built so that all the sympa-specific stuff is in a plug-in, and the hope is that someone will write plugins for other mlms. Actually, you can even get some of the functionality with any mlm. The sympa interface uses soap, and if another mlm also implements soap with some mostly compatible functions, then using other mlms should be "easy".

For me, the big barrier is that I don't know any python, which Mailman is written in, and Mailman keeps all its data in some kind of custom database, so I can't access it via mysql either.

This looks promising:

http://drupal.org/project/mm2

I think it means that there is now a way to communicate with mailman through http using xml-rpc, so with some modification of the soap-specific code, something could be done for a mailman plugin as well.

I haven't touched the og2mlm code in a long time, but from what I remember you should be able to create a module "og2mlm_mailman" that mirrors the functions provided within og2mlm_sympa. In fact, I would rename og2mlm_sympa to og2mlm_mailman and then delete the inner portions of most of the functions. They're primarily soap calls anyway. Then I'd use xml_rpc calls to mailman to make each function work. I'm sure it would take some time to get everything working correctly but it's probably not a huge endeavour, especially if you have some experience with mailman and/or xml_rpc.