Error Assertion Markup?

Hey
I was wondering if anyone has ever seen a project for passing around xml
instances with error assertions in the instances for testing validation.

The idea would be as follows:

The service would receive instances, process out assertions regarding the
validity of the instances, update a database with this validity information, run
a validation against the instance, compare the error report of the validation
with the asserted validity and return pertinent information to the requester.

As I'm building a demo of this I thought it would be good if someone already had
an error assertion markup instead of having to define it myself. 

-- 
Bryan Rasmussen