The Ethernet protocol was designed to adjudicate and route packets internal to one homogeneous, physical network. It handles media access, security, reliablity and routing in that one physical network.
What does Ethernet have to do with routing ?
It appears to me that your problem here is at the physical network level. You only have one physical network. Your Ethernet hardware should handle the routing to the proper ISP. If it were me, I would also expect my Ethernet hardware to be able to filter, load-balance and meter based on MAC address so that I could gaurantee no ISP could see another's traffic and accurately charge the ISP for bandwidth.
The problem you dont really have one physical network. We are trying to extend Ethernet into a logical network. The physical link to the customer is the same, but the customer is terminated logically. The reason that I like PPPoATM is that the end device requires littel intelligence , it knows one PVC, and all changes (linking them to the ISP of choice, propogate easily downstream).RFC 1483 (Ethernet over ATM, used in DSL and plausibly in cable ) is a great idea for business but not suitable for home users. Hardware that you speak of becomes just too expensive.
I think I hear you saying you must use hardware that does not use Ethernet. In which case, you have steered the conversation away from the original IPoE public network arena. You also have my condolences.
I strongly recommend you spend some quality time with the DOCSIS spec... Personally, I think DSL and wireless modems would benefit from using DOCSIS. I have not thought hard about this issue and am interested in other opinions...
It appears as if someone has thought of this before Someone is apparently doing this already (I don't know the name of the company), taking regular cable modems and using wireless for intramodem communication (why re-invent the wheel) instead of copper.
regards, fletcher