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 could use the SvnQuery project (http://svnquery.tigris.org) which has the intention to provide a query interface for doing such queries. It has a .NET API and a web frontend, providing the same query language. You can do complex queries with operators, nested expressions, wildcards, phrases and gap phrases. The operator you ask for, calculating the difference between now and the last commit date, is not implemented, but as it is an open source project you can either volunteer to do it or post a feature request for it :-)