Surprisingly Telegram-desktop (from devuan chimaera repo) works quite well on N900: messaging, calling, sending and receiving files work.
The app is slow to start (around a minute), swapping 100MB of stuff, and then works well. Ui is quite smooth.
Some non critical settings crash the app btw (it works finally with a longer click)
CPU usage is quite low and RAM usage is acceptable on N900.
With last Leste updates, that's perfectly usable without overclock.