All occurrences of image references in phpbb can (currently) be found with a fairly small number of regular expressions. I run several global search and replace operations to append timestamps to my asset urls in deployment. These timestamps allow me to cache assets indefinitely via expires headers (while serving them from the same vhost as the scripts).
I suspect it would not be too hard to transform relative paths into absolute urls, based on which file the paths occur in.
Now, this assumes you are not generating stylesheets on the fly via style.php. If you do, if my memory is correct you would need some less trivial changes to phpbb code to timestamp assets.


