I would NEVER go for that!
for one thing how long before someone hacks how it works and can suck all the bandwith for free and you are left with a bill and crappy internet ?
cable / isp / telco's do not use high end routers as they do not want to pay a lot for them and do not want a bunch of assets on the books etc.... so how are you going to make low cost hardware that has all of the features to make sure the home user does not get taken ?
no for the roaming devices they need to just work on better mobile network systems.
the telco's have a huge R&D and ROI to motiviate them to keep at it.
replace the telco router with a better model and run DD-WRT customized for the purpose of providing metered service to WIFI users. There is a boarding house above my office and the people staying there are always looking for WiFi to tap into. I would let people on the network if I knew I could keep them away from my traffic.