I took a look at the flights from CUN to DFW for tomorrow and they're showing a Terminal D arrival with "CUST" as the bag claim location. Based on that, it would seem that there's no preclearance at CUN and you'll wind up outside security at Terminal D. If you're looking for a meeting place outside security, the bar at the Hyatt adjacent to Terminal D might be a good spot.
Edit to add: here's a list of CBP pre-clearance facilities. CUN is not on the list.
http://www.cbp.gov/xp/cgov/toolbox/c..._locations.xml