These features will almost certainly be implemented in future versions of XXE:
An XXE configuration for editing RELAX NG schemas more comfortably[2].
A format plug-in for the compact syntax (probably based on Trang, an excellent multi-format schema converter written by James Clark).
include
and externalRef
elements in RELAX NG schemas are XML catalog aware, but the equivalent notations in the compact syntax are not yet XML catalog aware.
[2] It is already possible to open RELAX NG schemas (using the XML syntax) in XXE and automatically benefit from a full semantic validation.