No, definitely not bonded with the modems. At their end they have proprietary routers that just send IP packets down both lines, balancing them based on the sync speed of the lines.
They will happily sell you a similar router, to connect to two VDSL/PPPOE modems. I'm just trying to avoid spending the $700 they ask for their router (and I'd have rather have used pfSense if it was an option).
They are a very unusual ISP ;)
You can actually achieve much of what I want with three basic consumer routers. Use two with their own VDSL ports to route from the lines (both using the same WAN address), with a third router behind them doing the firewalling and NAT (with one of the other routers set as its default gateway). That doesn't get you upstream bonding though.
Anyway, thanks for your input, clearly pfSense can't meet my admittedly unusual requirements and it is time to try another route (dd-wrt if I can, if not my own Ubuntu build, failing that $700 router from ISP).