I do not think so. You may have seen some XTs with XTR top caps or even some prototypes leading up to Rapid Fires but I'm pretty sure they were never commercially available.
AFAIK they never existed in any form - Shimano had long set the course towards STI before the M900 gruppo. That said would love to be proved wrong and see some super secret prototypes!
yeah I didn't think they ever made production.
..definitely recall the 'XTR' top cap and seem to remember them being very nicely finished in M900 'titanium'... :?
I think one of the magazines mocked some up but I've never seen / heard of any otherwise.
I'd agree with John that Shimano wanted to push their rapidfire onto consumers.