Noob: XML validation capabilities beyond schemas

Hello,

Newbie alert.

So far, I have been validating my XML docs with RELAX NG schemas (I started out with XML Schema but switched to RELAX NG, which is much easier to read).  I use the XML schema datatypes, enumeration, lists, etc. to do most of the validation I want to do.  When I need to validate something that is beyond the capabilities of RELAX NG, I have been using GNU Awk to scan the XML and do further validation, but this is messy and error-prone.

Is there a better way?  Is it possible to add additional validation functions to parsers?  If so, how?

I am also investigating schematron.  How powerful/limited is this?

What is/are the standard way(s) to validate XML beyond the capabilities of schemas?  Any advice, pointers, or links would be appreciated.

Thanks for your help.