I recall discussing this idea with Bruce D’Arcus a long time ago (somewhere in the early 00-es, maybe).

The CSL files look like XML, but really they are more like a macro processing language. Parsing and interpreting that should not be all that hard, I expect. If I had to do this, I would convert the XML <macro> tags into actual lua functions while parsing the XML. The implied processing language is quite straightforward (unlike the stack-based postfix language in bibtex bst files). I don’t have time right now, but if someone reminds me in two months or so, I might have a go at it.