I've created an output format which consists of virtually standalone XML, with all external references imported into the document and all links made local (except for some obvious exceptions). This validates against tei_all and is now available alongside the old "standard" XML output, in which private URI schemes are resolved to file links. The latter output is still not quite valid against london_all.odd because of the constraints on the catRef attribute values; I'll have to think what to do about that. I might make the valList suggested/open for those attributes in the schema, but further constrain them in Schematron, which would allow for the enforcement without invalidating resolved references (since the output XML doesn't link to the Schematron).