While I was working on mobile theme for localtv I hit a bump with the following use case:
Ad server returns a chunk of HTML instead of injecting script into DOM, I also couldn’t make it as iframe source, since it’s just a chunk instead of full HTML documents. This led to a hack that @batmoo qualified as “pretty lame”: I make an ajax request that passes constructed URL to ajax action, that fetches remote URL and returns this piece of HTML.

Not sure of the best solution, perhaps, adding another hook like “acm_output_callback” or something like this.