Runako

If there is a bit of the piston protruding you can pull it out but the most important thing is not to damage the piston surface in any way. So padding or cloth around the teeth of pliers is one technique which has been used (emphasis on taking it very slowly so as not to damage piston). Otherwise put the piston back on the bike (or any connected brake line that takes the banjo). Might be longer and more frustrating but you're unlikely to damage the pistons.

Edit: never tried it myself but don't see any reason why you couldn't attach caliper to the rear brake as the banjo should be the same size. This is assuming the calipers are for the front and that you can't use the hydraulic system because you've released the fluid from the system.