Chris,
I am not positive about where the XACML standard is these days.
I would suggest that you contact the OASIS. OASIS is the consortium
that is handling XACML and SAML and is attempting to encourage the
implementation of PKI.

The first set of modifications was done to permit integration with a later
version of SAML.
You might want to contact contributors on Source Forge to determine whether
their Java
XACML demo has been updated. I am fairly certain that newer version of SAML
has
been implemented. I believe that Shibboleth, a middleware authorization
system, and SIGNET, a middleware privilege management system, use it.

If  I can find some students at Dartmouth, I would hope we can build a
new Policy Decision Process(PDP)  and a new Policy Enforcement Process (PEP)
that will be usable in a distributed system.  Ideally, the  PDP will permit
retrieving decisions from other PDPs (using SAML) and the PEP will use
decisions
from the master PDP (using SAML). This will require a PKI in place, but
Dartmouth has one.

If you find that there are missing pieces, please let me know!
Thanks,
Ed