Bluetooth keyboard is not possible ATM on N900 as bluetooth driver is kind of broken.

Telegram messaging works fine through Pidgin and even from Conversations.

However, Telegram-desktop seems the best way ATM since messaging, audio messages, video (laggy but working, file sharing, animated emoticons and calls (buggy like with many other devices) work on N900. It is just a bit long to launch (around a minute), then it works smoothly and can be kept in background.