There isn't a thing for that. It'd be difficult to make one because
stemming goes from unstemmed term to stemmed term, not the other way
around.
You could absolutely reanalyze the page on the fly and do highlighting that
way. I suspect you could do it with the analyze api as it stands right now.
I've heard folks ask for a freestanding highlight api where you don't have
to index the documents, just feed it a document and a query. That isn't
something that exists now though.
Nik