It would be fantastic to edit the same file as a neighbor in real time, à la SubEthaEdit, in Textmate. Coda does this. Is there a way to hack this stuff into TextMate?
I don't know of any collaborative editing plugin for TextMate. It used to be a requested feature right from the start but I think Allan said early that he didn't want it.
As for the hacking part, I would say you'd need access to the source which is impossible since it is closed. I don't think the plugin infrastructure is built with that kind of thing in mind.
Plus, what I imagine would be collaborative coding seems awfully nightmarish.