So has anyone ever tried to build telepathy-butterfly for Maemo? It seems to me that such an attempt would be far less complicated than telepathy-haze for MSN support, since Haze requires ridiculous dependencies like a 20MB download of pidgin-data.

I don't have Scratchbox working, but I would try it if I did

Info available on the Telepathy wiki:
http://telepathy.freedesktop.org/wiki/Components

Scroll down a bit and you'll find some links about Butterfly under "In Development". It works fine for me on my Ubuntu desktop, including multi user chat support.