In principle it's the same as quinn's answer, but as a working script instead of separate commands which need adaption for each machine/usage.
I don't know about the overhead in this, it seems to me like it encrypts/decrypts everything twice.
# Reverse sshfs. You need ssh servers on both ends, the script logs first
# onto the remote end and then back into the local one
# Usage: sshfsr dir [user@]host:mountpoint [options]
# [options] are passed on to the remote sshfs
REMOTE=$(echo $2 | grep -o '^[^:]*')
REMOTEPATH=$(echo $2 | grep -o '[^:]*$')
ssh $REMOTE -R $PORT:localhost:22 "sshfs -o NoHostAuthenticationForLocalhost=yes -p $PORT $ARGS $LOCALUSER@localhost:$LOCALPATH $REMOTEPATH" &
I disable HostAuthenticationForLocalhost because obviously localhost can be anything. This is perfectly safe with public key authentication. You shouldn't use passwords anyway, but even with passwords you are connecting to a host you know you control.