I have another rumour that tends to confirm the holodeck theory; it is a quote from the set -- "Throughout the eight days of shooting, Frakes was at various points dressed in a TNG-era uniform; as an NX-01 crewman; as a MACO; and ... as one other person who perhaps should remain a surprise."
More here: http://www.startrek.com/startrek/vie...icle/9996.html
The other person who perhaps should not be named... I've read a rumour that Riker turns out to be 'Future Guy' who talked to Silik (we never did discover who he was!)
Another is that he simply turns up as 'Chef'!
These rumours are great fun, and probably much more creative than the final product ever will be