David Attard has a couple of posts giving more detail/examples on what you can do with the advanced search customisation technique I posted earlier.

 

When debugging this sort of customisation, you need to make sure you stay logged on as the same user - A couple of times I have spent a while trying to figure out why a small change to the search query produced a completely different set of results only to realise that I had switched to a user with no access to the missing documents.

 

I’ve been working on a web part that creates the necessary fields based on slightly simpler XML and XSL configuration properties, much like the out of box search results web part. Hopefully I’ll be able to release that at some point.