I have the use-case to be able to internalize URLs again on the author side. I had hoped that there would be a feature for this in URLHandler, seeing as the OOTB AEM externalizer also doesn't support this without having the /etc/maps available on author side (which is not what we want).
Is this already possible, and if not, would it be hard to achieve this?
no, the url handler currently does not support this.
the url handler also relies on the /etc/maps configuration for sling mapping-based url rewriting.
you could use ResourceResolver.resolve for this use case to get the matching resource, but this also requires the fitting /etc/maps configuration in place.