Caliper pistons are normally sealed with an O-ring that has a square cross-section. This O-ring stretches when the brakes are applied and the piston extends towards the rotor. When the pedal is released, the O-ring relaxes and retracts the piston. Because the rotor and brake pad surfaces are flat and aligned it takes very little movement to obtain pad-to-rotor clearance.
Apart from issues of clearance, protection from debris and damage, and ease of bleeding, there is no reason you can’t mount a brake caliper at any position around the circumference of the rotor. The 3 ‘o’ clock and 9 ‘o’ clock positions are the most common (i.e. in front of, or behind the rotor), as this affords good protection and allows the caliper to be mounted with the bleed-screw up (highly recommended), for ease of bleeding and a better proportional brake system. This is easily achieved with the DEEMAXX EXCLUSIVE 120 degree bleed screw mounting. Mounting at 6 ‘o’clock would almost certainly make the caliper far too vulnerable, rob ground clearance, and imparts no particular advantage. However, there’s no reason you can’t mount the caliper at 12 ‘o’ clock, provided you are prepared to remove the caliper (to orient the bleeder screw up) to bleed the brakes. If you do, be sure to insert a block of wood or other spacer in the caliper while bleeding.