Theoretically, yes -- anyone can be denied entry for any operational reason, even with a valid ticket admission and reservation.
As my esteemed DIS colleagues stated above, in the
Before Times™, Disneyland would start by suspending one-day park ticket sales, and proceed to outright blocking entry into the parks until those already inside start to exit (usually, folks would bide their time at DCA -- I don't ever remember DCA having to shut its gates).
With the reservation calendar in place, I really highly doubt that they would do something like allow the # of reservations to be greater than the actual, physical, legal, "fire marshal" capacity, so I would be confident in being able to get in.
Caveat being... stuff happens, keep an ear out on the forums for reports, plan on getting there early (if anything, because lines become unbearable in the afternoon), and be flexible. Disneyland between December 15-January 5 is a wild, wild place.