Thanks flexvault... I had a similar idea while pondering all this.
Our application has a lot of file uploads, so many requests need to read several MB from STDIN -- which would complicate internal handoff. I was thinking I could have a script like /fast.pl which takes all GET requests and proxies them to a listener, as you describe, and then another script /upload.pl which accepts HTTP file uploads as traditional CGI.
Anyway, it is reassuring to know that somebody has actually done this and had it work well. We don't have mod_perl but perhaps I could just take the hit of running the 200-line stub as normal CGI, or I could write a persistent C# stub.