I was wondering if it is now or will soon be possible to make a Lightning payment through which the payee discloses an arbitrary secret to the payer, provided the payer has a hash of the secret beforehand and can only get the secret when the payee is paid. The payee must not be able to withhold the secret if they are paid. I'd like to hear if anyone knows of a way to make an exchange like this using Lightning, or if this is really only feasible with an on-chain script?