Here's some info for XML Schema users!
As refered on my earlier blog post (ref, http://mukulgandhi.blogspot.com/2010/04/xsd-11-xsprecisiondecimal-assertions.html), XSD 1.1 assertions error message was not a standard feature of the XML Schema language spec, and XML Schema WG was deliberating on this issue at that point of time.
As of now, the XML Schema WG has taken a decision on this, which is as following (which is an response from David Ezell, XML Schema WG chair):
The WG believes that this topic should be covered in a separate note describing best practices for how to handle this issue. Liam suggests following the i18n practice of publishing "articles" to recommend best practice.
See http://www.w3.org/International/ for examples.
Therefore, keeping in view this observation from XML Schema WG, the Xerces-J implementation would not change about assertions error messages. Or unless, there are any specific comments from Xerces users about assertions error reporting.