I have a pretty short carpet (average apartment type carpeting) and the casters on my cab stick out a little less than an inch below the bottom edges. With the full weight of all the guts installed, it looks like the edges are resting flush with the carpeting.
I have locking wheels on the front, but once it settles into the carpet for a few hours, the locks are unnecessary. If needed, I can reach under with a large screwdriver to trip the brakes, but the weight of the cab is more than enough to keep it from rolling even an inch during play. It's still fairly easy to roll it around, but it takes a good push to dislodge the wheels from their impressions in the carpet! If you want to cut holes to access the locks, why don't you cut them in the floor of the cab and operate them from above, rather than marring the sides of your cab? You wouldn't be able to operate both of the brakes from holes in the side panels anyway, since both would have the brake on the same side, one of them is going to be facing the inside of the cab...

BTW, I used the Happ 3" cabinet casters for the rear of the cab (the ones with the L-shaped bracket), and some 4" heavy duty swivel casters for the front. It was necessary to make a 4" recess in the front of the cab to allow the swivel casters to turn freely. The front casters are actually mounted to the floor of the cab, and an additional sheet of 3/4" ply was screwed to the two 2x4 beams that form the bottom frame of the cab. Slots were cut in this plywood and in the back kickplate of the cab to allow mounting of the rear casters. Both sets of wheels protrude about 0.9" (a little more than 7/8") from the bottom edge of the cabinet.
