I've been thinking about it this morning. I think the solution to the backstop would be to solve the leverage problem, which is basically that, if activated, it creates a tolerable situation for the EU but a potentially intolerable one for the UK, which skews the future trade talks unfairly.
If the UK is to sign up to an indefinite backstop (which keeps it in the customs union), then there should be something added to the deal which makes remaining in the backstop long-term uncomfortable to the EU as well. That way both parties have a real incentive to get the trade negotiations finalised and exit the backstop arrangements, and it wouldn't be relying on good faith.
It's the MAD doctrine basically. The backstop needs to be equally problematic to both parties to ensure we don't get stuck in it.
What is particularly intolerable? And do you think that the EU wants to have anything to do with a joint committee on the long term?