Thank you for that link. I hope nobody minds the thread-hijacking to ask a little more about this - it's not a topic that seems to have much Google juice.
With that setup, do I understand correctly that - assuming the ISP has the bonding set up on their end - configuring the TEQL interface as you have will work regardless of the connection method used? It looks like it's actually very simple so long as you know the secret sauce.
What I'm not sure about is the IP addresses on the routers. You say that they need to support two different *LAN* addresses; how are the WAN ports configured? Are they bridged or do they need an additional address?
Say for the sake of example that you have a /29 netblock, giving you 6 addresses once you've accounted for the network and broadcast addresses. You use one that's shared between the routers and one for the teql0 interface, leaving 4 available for other machines. Is that correct?
Incidentally, would you recommend the Zyxel P-660 series? I do wonder if a better modem/router might solve my periodic loss of ADSL synchronisation, and while I've tried two different models, it's entirely plausible that they're both fairly intolerant of noisy lines.