My suggestion would be to set up your own prosody XMPP server (I did this last week), and set `tls_profile = "legacy"`, this way your N900 should be able to fully connect. I have it working for chats, group chats and calls on Fremantle.

Of course you won't get OMEMO, but hey.

EDIT: To be clear, you can just use the built-in conversations, which uses telepathy-gabble.