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
Apart from using a Subversion command-line or a graphical client such as TortoiseSVN to view svn repository history, you can install an advanced web interface for Subversion repos.
For example, in VisualSVN Server 3.2 and newer you can use a web-based revision history viewer, see the demo here.
See the description of the HTML5-based web svn client here.