Is there any reason I can't or shouldn't use a ribbon cable to connect either the TFT module to the shield OR to connect the shield to the arduino mega?
I got a really nice alarm system box with hinged door and would like to mount the TFT remotely from the arduino. I suspect the cable length to be about 12".
Yes you can connect the display to the shield with a ribbon cable (IDE extender 40 pins/40 wires). 12" isn't a long distance so I think you won't have any problem 
I have a suggestion and that is to look for an oldschool 40 pin CD connector cable, most of the 40 pin ide cables have one pin plugged, Ive never tried to remove it... don't think I could with hand tools But I have much the same issue, I have mine separated with a 40 pin M/F cable I bought from Electrodragon for about $2.50. There are available 2.54 mm dual male pin strips 5.65 mm or .225" that can make a male adapter on one end then you will have a 8 to whatever inches works long cable. just make sure that one in the middle isn't plugged, that cable is an IDE cable. I dug in my junk cable box and found 2 or 3 and I paid 11 cents a piece for the 40 pin male to male strips from the same place. Buy a bunch as they are great for interconnecting female jumpers from breadboards to external things like GPS or keypads...
Bob
Thanks Guy's !