I found some people who shared the problem but either they did not solve it or they never told anyone.
So I gave up my search for something that was already there and started hacking away my own code. The excellent Mongrel RDOC documentation made it clear that creating a Mongrel request handler would be a snap and after realizing this, I just had to port some of the code of Ruby’s standard library’s XMLRPC module CGIServer over to a Mongrel Request Handler.
The result is Proof Of Concept (.rb) script that allows to run a XMLRPC Server in Ruby on top of Mongrel. I’ll cut out the interesting code, add some tests and make it available for download over the next days (hopefully).
Maybe this will find its way into Mongrel’s or Ruby standard library’s code base one day…