Working on the ODD to allow teiCorpus (which was trivial), but then spent a long time trying to get a schematron rule in place to check the attributes of a processing-instruction. Note that XSL variables do work in schematron patterns, but functions and other bits (at least with the current stylesheets) do not.
The second issue was that the schematron building code (that I lifted from MH's Keats code) does not automatically process "foreign elements" (i.e. XSLT variables) in the final XSLT step (iso-to-svrl-2.xsl). You have to add an "allow-foreign=true" parameter to the XSLT for it to work as expected.