from dom0, as root, you could xm list to see all vms and xm console to plug right on to the console of any VM
Say you did xm console udom - that would put you on the CLI console of CDOM and you could root in, basic linux CLI to change default gateway, service network restart, then you might be able to get to the HTTP page again to set it in there properly. CDOM triggers scripts in dom0 when you do management stuff and dom0 then triggers scripts in the VMs - like when you change a default gateway in that webpage, then CDOM tells dom0 to tell all VMs to reconfigure themselves accordingly.
You could try that.
Also, there's a backend private network as well. 172.20.10.x if I recall correctly. If you ifconfig in dom0 you'll see virtual interfaces - at least 2 per vm - one on the private backend management network and one on the corp lan. So, cdom should have an IP on that. If dom0 is 172.20.10.1, try ssh'ing to 172.20.10.2 - and if you can ping/ssh from dom0 to cdom on the private interface, then you could build an SSH tunnel
To build and SSH tunnel, ssh on over with putty to dom0. Right click, settings, down at the bottom, ssh and tunnels. Leave the radio boxes "local" and "auto" selected. Put sourceport 443. put dest port 172.20.10.2:443 and click add underneath and then click apply - if you don't click add before apply, it doesn't take. Once you do that,
goes on the ssh tunnel and dom0 forwards it to the destination. Might find your way into the webpage that way.
