I don't know if you learned any more on the subject or not, but I too am looking at the same and worse.
I have streaming server locations intended for bonded serving on ADSL and some of the clients receiving will require bonded POTS dial in coordination.
ISP's don't like to discuss the matter too openly, but for those who do not have cable or DSL, they want to push ISDN, which is in fact a reverse multiplexed with ppoe. Problem is that while ISDN service is competitive, hardware is not and cramps the users pocket for installation.
I have an ISP that's willing to let me work inside their dial in setting and will devote a couple of dial in accounts for a price, but I have only today begun speaking with them, to set up a framework for ISP reseller privilage, finding them on the fence with it so far, until I provide them some statistics on market study.
If you're serious in the approach, we might find areas to help one another, if you're still watching the post, I'll flag the thread for notification.