I'm guessing that using a Input or Output filter handler would be a better option ... combined with the standard Apache modules proxy and proxy_http to avoid reinventing the wheel.

It's certainly an option and likely one which involves less work on your part. You should essentially just be writing one simple(ish) filter handler (an Input Filter from your description of the problem) which you could test in isolation and then just the small matter of hooking it in early enough so that it acts before the proxy module gets to the request. I haven't tried this combination before but you should be able to rig a very simple test (eg. with a basic text-subtitution filter and a backend which just echoes the data) as proof of concept.

I know mod_perl2 is not the "sexiest" framework at the moment

It's always good to have one eye on the latest hot thing. 95% of them will come to nothing but the rest will form the next generation of de-facto standards. Nevertheless, it is a foolish monk indeed who forgets or disregards the established magic in pursuit of ephemeral fads. I recall being blown away by mod_perl when it was first shown to me and it is no less astonishing and powerful today. Good luck with your project.