This is a rather low-tech solution, but one that I've used several times where incompatible systems needed to be strung together: invent a spurious confirmation page.

Eg. instead of exposing people to the execrable interface offered by the credit card handler that amnesty uses, I've created a set of (gratingly cheerful) forms which culminate in a 'please check and continue' page, the sole purpose of which is to assemble a hidden form which presents the data to the remote system.

(As with any such intervention - including your LWP approach - there's still the problem of tracking changes in the remote system.)

.2p