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 sub
You can use svn log command to produce an XML file with a lot of information about all commits like this:
svn log URL --xml --verbose > commits.xml
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.