Those are interesting ideas and it probably would not be difficult to create a patch if you were interested, but I'm curious: What about XSL makes what seems to me an elementary string-processing task so difficult?
Well, XSL 1.0 (which is the one that "comes for free" with Solr/java) doesn't handle dates and times at all. XSL 2.0 handles it well enough, but it's only supported through a GPL jar, which we can't distribute.
It's more than string processing, anyway. I would want to convert the Solr Time 2007-03-15T00:41:5:2Z to "March 15th, 2007" in a web app. I'd also like to say 'Posted 3 days ago." In my vision of things, that work is done on Solr's side. (The former case with a strftime type formatter in solrconfig, the latter by having strftime return the day number this year.)
