Thanks Sebastian, and everyone! The software we are using--FontoXML--is really great, but has not been around very long, and as I said they plan to support Relax NG soon. But I do think
it's useful to have the option of producing an XML Schema, as many people still feel more comfortable with it.
The word "still" there implies that relaxng is a fresh faced newcomer, and xsd the hoary old standard. They're actually about the same age.
There are interesting reasons to use xsd or dtd rather than rng, but tradition isn't one of them...
I have, I hope, trapped the situation you found and will release new stylesheets soon.