I've thought about this before as a forum feature. It would be kind of cool, but I haven't done it for these reasons:
- I predict that people won't use the feature very much. Donation addresses in signatures rarely receive much BTC. (Reddit's bitcointip will eventually lose popularity, I think.)
- If the donation amount is displayed, something needs to prevent people from inflating the number by donating to themselves.
- Running a Bitcoin client requires quite a bit of work to make sure the client is updated, it doesn't die, most of the BTC is held in cold storage, etc.
- I don't want anyone to use the forum as an E-Wallet. That's not the forum's purpose, and there might be legal risks associated with doing this.

Bots can't be posting in threads because such posts would be off-topic. Any posts by humans consisting entirely of bot commands would also be off-topic. Bots can use PMs as long as PM recipients have some way of unsubscribing from all of the bot's PMs.

I've been thinking about creating a forum API to make it easier to create bots, collect stats, etc. I need to think carefully about how best to do this, since I want the interface to stay the same when the forum moves away from SMF.