Sorry just noticed yours is a 2013....does it look like the one in the photo? It might be easier to rotate the piston.....or see if you can remove the round thing (bellcrank) that the cable is attached to, then turn the shaft.
If you tried a "C" clamp I will assume (risky as that is) you have removed the caliper and park brake cable. On the large plastic disc where the park cable connects you have a nut in the middle. Remove that nut,14mm I believe, and remove the disc this will allow the park brake to fully release. Now if it does not let you push the piston back it may be cocked at an angle (don't force it). If lines are still connected place a block of wood or old pads and a spacer then pump the brakes just enough to move the piston out a bit then try pushing it back. Must be even....
2012 RS sm5 , 998cc V-Twin 106hp DIY brake and park brake Classic Black