schematron for xml schema validation

Hey
I'm doing a little schematron for validation of spec required xml
schema structure, some of the validation rules cannot be done by
schematron itself as it involves doing validation across external
documents (I am focusing on a schematron that will work in all
implementations so use of document function etc. not doable.)

Anybody have experience with this problem, ideas for controlling the
schematron elegantly across a set of schemas?