Hi Jens,
I think it's been done before - see the discussions in
https://github.com/ndmitchell/shake/issues/26 - but I don't really
know the details. Those discussions lead to the API's newThrottle and
unsafeExtraThread, which may be useful to you.
You can define your own Rule class and avoid having local files, but
I'd probably use dummy local files to start with, get it working that
way, then define your own rules later.
Thanks, Neil