Currently I would simply like to find all files that have not been modified in the last X days but ultimately I would like to be able to make more complex queries against my subversion repository.
Is there a Subversion Query Language of some sort or an API that I could use?
You can use svn log command to produce an XML file with a lot of information about all commits like this:
There are some more options you can play with to limit the revision range, get more information on rev props, include merge info, etc.
The problem then becomes ‘how do I perform queries on the content of an XML document’, which is easier than working with the existing SVN APIs. For example, in C# you can do LINQ queries on XML.