The resource you are referring to can only return all the documents in the file cabinet. The "q" parameter seems like it should work to act as a filter, but IIRC, it does not work, failing in both the .NET query and the straight URL resource type of query.
To get queries working via the Platform SDK you need to go through a search dialog, using a resource such as:
POST the query to that URL using post data that has the proper XML syntax to define a "DialogExpression" node.
What I really wish the .NET library offered was a way to see what the POST data and the resource URL looked like at the time you execute a method via the DocuWare wrapper libraries. That would be the ultimate in educational. As it stands, the .NET wrappers hide the underpinnings too well, so when you need to access the same Platform functionality from outside of .NET you need to do a lot of trial and error until something works.
I guess the .NET stuff does show a lof of self-relation URLs, and those can help narrow down what things should look like.
Let me know if you need an example of XML that comprises a DialogExpression. Alternately you can use JSON if you are more comfortable with that (I have always had better luck getting XML to work as opposed to JSON...).