Just tested Qt Mobile Hotspot + sshuttle: it doesn't work for now.

Qt Mobile Hotspot messes iptables rules by flushing everything when you start it or stop it (not needed I think)
Entering manually the rules that sshuttle create, let the N900 use the tunnel again but other devices can't access anything.

I don't know anything about iptables for now but I will try to find how we can make this working and eventually make scripts to manage it more easily.