I was recently asked about Exist DB support for PicketBox XACML (formerly JBossXACML) mainly to retrieve missing XACML attributes during policy evaluation. The question was asked by one of our beta testers of PicketLink (http://jboss.org/picketlink).
We now have ExistDB integration support for our XACML Engine. You can read about it here:
PicketBox XACML Integration with Exist DB
I think storing xml files (policies) in an XML native database is a fine idea as long as it is a small set.