You can log into an @gmail.com Hangouts account through XMPP. The instructions on https://wiki.bitlbee.org/HowtoGtalk worked for my account which is also accessed by Hangouts on Android L.

(At least person-to-person messaging works well - I have not attempted group chats or video or stuff)

Indeed. Google voice texts, group chats, and video stuff do not work. I get incoming audio chat requests on pidgin for google voice calls, but they don't work.