That's the trick, thank you!
For reference, ::xo::cc set_parameter template_file YOURTEMPLATE should be called after package initialization, or ::xo::cc instance won't be available.
For my convenience, I have slightly changed index.vuh for xowiki on my instance, so it accepts also a "template_file" parameter. This way I can force the resolver to use the template I want when viewing xowiki pages, case by case. It's a poor man solution, but does exactly what I need and doesn't harm the package whatsoever.
Putting xowf in the mix could do, as I could say: "use this template when logged and another one otherwise", but probably is a bit too much right now. I will get there some other time.
Thanks again!