I wouldn't hold my breath. XML processing hasn't received any love in some time.
I process all my XML in an XML editor. I would recommend doing the same.
There are only a very small number of XML users in InDesign. It's pretty unlikely that there will be time for the programmers to provide that support. There are a ton of much more important and popular features which will win out, not to mention the number of outstanding bug fixes which are waiting.
While I agree with Steve's basic premise, it is a shame that with InDesign's plugin architecture they can't make a convenient way for a user to specify their own XSLT processor for import/export. -Rick