I'm interested in the answer too, Smithy. I still remember the prickly pear creme brulee we had at the Farview Lodge restaurant in Mesa Verde many years ago. Using the juice in place of all or part of the sugar might be worth pursuing.
But now that I think about it, probably the juice could be reduced and then mixed in with the custard. Or maybe with the sugar? My memory of the creme brulee was that it was in the custard.