Could have used this years ago, but at least I know about it now: mod_xsendfile

mod_xsendfile is a small Apache2 module that processes X-SENDFILE headers registered by the original output handler.

If it encounters the presence of such header it will discard all output and send the file specified by that header instead using Apache internals including all optimizations like caching-headers and sendfile or mmap if configured.

It is useful for processing script-output of e.g. php, perl or any cgi.