[Sorry, have not read whole thread.] Martin, are we meeting tomorrow morning? If so, I think this should be on the top of our agenda. (Unless, of course, someone else has fixed it by then, for which I keep my fingers crossed. :-) I am hoping to have 1-2 hours Fri afternoon and anywhere from 0-3 hours on Sat afternoon to devote to this problem.
We did explore this issue last Friday at the XSLT Group call and the problem is related to how simplePrint's ODD is structured: it uses specGrpRef to declarations that are not children of schemaSpec, which is totally fine, but the processing gets confused. In particular we noticed that some references to attribute classes get duplicated in the RNG. We were not successful in fixing this last Friday and I'm not 100% who's in charge of fixing it soon (and I'm not volunteering because I'm swamped, but will follow orders if compelled). The strategy we were trying was to move the specGrps inside the *right* schemaSpec before processing further.
For further reference, this issue seems to have originated after changes to support multiple schemaSpecs.