I should be able to get the prolog and comments into the data object
I was optimistic. I misunderstood that the handler return expectations would match the rule return expectations, and whatever was returned from the handler would be added to the data structure, similarly to a custom rule. That didn't happen. And with haukex's advice that round-tripping is difficult, this might not be the right path after all. If anyone has advice for continuing down this path, I will definitely experiment with it, no matter which direction I go from here.