Common API for grammar-constraint

Hello,

in a xml-editor context, I'm looking for an API which could give the 
edition-possibilities on a node ( and a caret position), according to a 
given grammar.
The idea is to fill the completion popup in an editor, to allow/forbid 
adds, deletes, moves, etc...

I want to use many grammars (RelaxNG, XSD, Schematron), so I'm looking 
for a tool/API which is common, or a unified result format.

XSOM provides an object model from a XSD, which is interesting, but does 
not support other grammars.
I have no idea on how to validate a document-fragment (with a context, 
if needed), I've found no API for this.

Any hints on these subjects will be much appreciated.

Best regards,
Christophe