Mosh opens an ssh session to setup the initial connection. Keeping that open would be a good way to maintain port forwarding while still using mosh for interaction. I've tried setting this up without any success:
mosh --ssh="ssh -fN" # hangs forever mosh --ssh="ssh -f" # ssh gets terminate
mosh --ssh="ssh -fN" # hangs forever
mosh --ssh="ssh -f" # ssh gets terminated