I summarized my thoughts on this subject in a wiki page, http://wiki.oasis-open.org/xacml/AttributeAssertionsInRequestContext .
I implemented a minimal policy evaluator in XSLT to prove the concept of polymorphic comparison functions to evaluate either attribute values or attribute predicates in the request context.
When it is a little further along I will share it with the group if anyone is interested.
Although this introduces a bit more complexity, and a lot more room for indeterminate policy evaluation, I think it would be useful extension to XACML.
Regards, --Paul
