I have achieved this using a VPN. You need to setup a VPN between the zeroshell box and a host on the internet. You can then connect back down the VPN to the zeroshell box via the host. The only problem is if the VPN drops out, and doesn’t recover, you cannot get back into the box.
The other option would be to setup a persistent SSH tunnel from the zeroshell box to a host, and use port forwarding over SSH to achieve the same result. I guess this is a similar concept to the VPN, but I used the VPN option.
Hope this helps!