I've decided to alter the .xsl file that generates the .wod string to assume any WOHyperlink, WOForm, WOImageButton etc... will be able to accept a ?theme=x parameter.
It seems to work well, but it also means that links to external pages will have a url parameter tacked on that could either be ignored (hopefully), or possibly misinterpreted.
I don't expect I'll be linking to external CGI scripts, and other pages aren't likely to read parameters from the URL, so it's a risk I'll be willing to take.