I've looked at reverse ssh tunnelling, but I think what most people really want is ppp-over-ssh.  Reverse ssh tunnelling presupposes that the firewalling allows ports like 19999 through.  In many cases it's going to be better to go exclusively through whatever port is being used for ssh, 22 or otherwise.  An example of ppp-over-ssh would be:

 sudo /usr/sbin/pppd updetach pty \
"sudo ssh 138.47.99.99 sudo /usr/sbin/pppd notty 192.168.254.254:192.168.254.253"

which provides a ppp0 interface on each of the two machines.

--Fred Feirtag