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
Since 1.7 SVN, the working copy has a wc.db file in the .svn directory. It is a basic sqlite database, so pretty easy to query.
You should make sure your working copy is up-to-date. If you are doing anything more than a quick and simple select, it would be safer to copy the file rather than risk breaking it.
The change date is based on the unix epoch. For example:
sqlite3 .svn\wc.db "select changed_revision, datetime((changed_date/1000000),'unixepoch') changed_date, changed_author, repos_path from NODES"