Would the auth information showup in the AT&T modem admin page? They doesn't use PPPOE, just standard DHCP. Just hoping they don't filter by Mac address upstream.
Interesting. If they do use the device MAC address, that could turn out to be really easy. :)
Mac address spoofing is super easy (basic admin task almost) on most *nix's, though (from hazy memory) it does depend on the network card capabilities and driver.
Looking at the general info page for the EdgeRouter X, it seems to run something called "EdgeOS":
On (V)DSL, they use 802.1X, not sure about their fiber service. That's what forces me to use their "home gateway" (since it auths using the certificate in the device).
As a curiosity thing, have you ever cracked open the device to see what's inside? Maybe JTAG ports on the main board, or even having the cert stored (say) on microSD card? :D