I'd probably start with a configurable bot with a few predefined plugins (RSS feeds, version control / bug tracker integration, etc) and a configuration UI (similar to Github's Services hooks)

Eventually you could add a way to define custom plugins that are properly sandboxed.

You're describing http://hubot.github.com/ which ist pretty much a configurable bot with a few scripts and an irc adapter. It doesn't have a configuration page though.

I would suggest something like Twilio where you host your own xml at a specified url that twilio calls when it receives a call/sms for you. Similarly grove api consumers can specify a url that can be called by grove when a message is received in a channel and api consumers can respond ina predefined json.