Before I reinvent the wheel, does anyone know of an XML schema for
representing systematic/taxonomic/nomenclatural information?

I'm not finding any documentation for Gilmour's Taxonomic Markup
Language, and on first sight PhyloXML doesn't seem to offer the level of
support I'd want for nomenclatural information. (But could be modified
for my purposes.) Taxonomic Concept Schema seems more promising.
--
Stewart Robert Hinsley