I did actually prototype multi-link PPP over multiple GRE tunnels as a way of bonding multiple links, with (some level of) redundancy, with the intention of offering that as a product at the ISP I worked. The idea was, basically "up to four ADSL links, possibly from multiple providers, each used as a member of the MPPP bundle, then dial-up of some sort as last resort".

Also tested ADSL as a bond/fallback for a leased line, around then. Ah, the days...