Caneris/Acanac were the only ones who ever implemented MLPPP over disparate connections. My understanding is that they basically run a PPP session over a VPN on the cable line, and bond that to the PPP session from the PPPoE session. This is all custom code.
Considering the lead developer quit Caneris quite some time ago, I'm not sure that they're still working on this. If they haven't, you're SOL; your only other option might be to run PPP-based VPNs on all connections and bond them (like IPSEC or PPTP). Nobody really supports bonding connections with different speeds very well (that was part of the Caneris/Acanac solution), so you'll be stuck in lowest-common-denominator territory.--
Developer: Tomato/MLPPP, Linux/MLPPP, etc »fixppp.org