instructions

1. Put these files in /etc/persistent on the ubnt
2. on the ubnt device, do:
       cd /etc/persistent
       chmod +x rc.postsysinit ssh-tunnel
       dropbearkey -t rsa -f ubnt-rsa
3. dropbear will print out the public-key part (line that looks like: ssh-rsa XXXXXX... )
4. put this ssh-rsa line into ~/.ssh/authorized_keys on the server
5. on the ubnt device, do:
       cfgmtd -r
       cfgmtd -w -p /etc
6. reboot the ubnt (eg type "reboot" and enter)

Now, on the server (in this case xylem.cooperi.net) you can do 
ssh -p 12345 admin@localhost
type the admin password for the ubnt device and you're in!

Note: this is a simple draft, it needs support for dynamic port numbers, needs to handle timeouts better and have some sort of mechanism on the server side to keep track of which ports belong to which APs

rc.postsysinit

#!/usr/bin/sh
/etc/persistent/ssh-tunnel &

ssh-tunnel

#!/usr/bin/sh
trap "echo hup" 1

while true; do
  ssh -R 12345:127.0.0.1:22 -y -N -i /etc/persistent/ubnt-rsa ubnt@xylem.cooperi.net 
done