With adsl, I'd just discard the isp device and use a cheap modem-only device.
As I said docsis is often MAC address controlled, so if you can't get the admin password to unlock the devices (http://portforward.com has lots of isp configs/auth details) you could buy your own docsis modem/router that supports configurable MAC address and simply spoof the isp provided device.
For fibre based devices usually you just have a ONT with an Ethernet port, so no modem required just plug in your router.
If you can give more info on the specific last-mile tech you're dealing with, we can probably make some suggestions.
Fibre to the home. I think your ONT is my NTD. Box on my wall that I'm not really allowed to touch, then Ethernet straight to my router which handles PPPoE.
I've never seen/used this type of thing before, but then again I've never found my need for mobile data that massive that regular 3g/4g wasn't a reasonable option.
Maybe thats just me though. My Thai phone provider gives me global roaming with unlimited data capped at ฿300 a day (about $10 a day). The last time my wife and I came back to Australia together, we had pre-paid Telstra sims. The time after when I came alone for work, I didn't bother, as the roaming cost worked out better than Telstra's shitty pricing, and I still got to use their network.
As for an actual solution, I doubt there is one an ideal solution in this case. Honestly, I would want to need access to a lot of other people's Wifi before I signed up for this type of deal - how sure are you this doesn't give the other Telstra members carte-blanche access to your local network, and how sure are you a judge will accept "but your honour, that could have been anyone downloading those pictures, I have Telstra Air".
I haven't had access to the Thomson gateway device for long enough to see how the traffic is segregated. If I do end up changing to Telstra I'll have a look, although I'm not sure I'd be allowed to solder up the serial-out on their device ;)
$10/day is $300+ a month - I pay $50/month for 7GB of data on Telstra's 3G/4G networks. I haven't actually needed to use Telstra Air because the middle of the city has free wifi by iiNet/Internode (RIP).
Which ones? Sometimes the configuration is confusing, but I haven't found one I can't disable yet (my Ariss/Moto SBG5580 doesn't have a simple 'bridge mode' toggle, I have to disable NAT/NAPT to put it into bridge mode).
As I said docsis is often MAC address controlled, so if you can't get the admin password to unlock the devices (http://portforward.com has lots of isp configs/auth details) you could buy your own docsis modem/router that supports configurable MAC address and simply spoof the isp provided device.
For fibre based devices usually you just have a ONT with an Ethernet port, so no modem required just plug in your router.
If you can give more info on the specific last-mile tech you're dealing with, we can probably make some suggestions.