OP: I'm not sure about that particular issue - we tested on a D5 site and it worked fine, though maybe some issue exists on D6 that we haven't gotten to yet. The role promotion expiration shouldn't assume any sort of trial period.

Regarding subscriptions - you're correct. It doesn't mean it's impossible, because there is a hook getting called when a fee get's cancelled... it's just that right now it doesn't directly tie to other things like role promotions.