My understanding from this pull request Ensure cooked CSS/JS resources are treated as "stable resources" for caching by alecpm · Pull Request #1174 · plone/Products.CMFPlone · GitHub
is that the ++unique++ traverser allows generation of unique "cacheable" resource paths.
It works for theme resources:
++theme++my.theme/++unique++xxxx/style.css
and for the plone static resource namespace.
++plone++static/++unique++2018-02-09%2011%3A16%3A34.940752/faceted-navigation-jquery.min.css
Is there a design related reason why it does not work for static files in filesystem based add-ons?
For example:
++resource++my.addon/++unique++xxxx/txbizlaw.css