Sorry, I got my acronyms mixed up. Oxygen had support for NRL (Namespace Routing Language) which became deprecated and was replaced with NVDL.
We never had support for NRL (Natural Rule Language), I started looking into the link you provided, indeed it looks a lot like Schematron, actually they seem to have an article which seems to show how to convert it to Schematron for use with Oxygen:http://nrl.sourceforge.net/tutorials/schematron/tutorial.html
I will add an internal issue to find some time and look further into this language.
If it were better integrated in Oxygen, how would you see that integration? Have some way to state that an XML should be validated with such a NRL file and then show the validation errors just as we do for all other schemas? Maybe a specialized NRL editor with specific syntax highlight?