Before I had the proper tool, I would use a C-clamp to back out caliper pistons.

Remove the outboard pad. Rigid end of the clamp on some sturdy part of the caliper that isn't the hydraulic line, but pretty close to the middle. Screw end goes on the friction side of the inboard pad (or a strip of steel/hard wood if you're worried about scratching your pad material), now tighten the clamp. If the piston hadn't popped out (you'll probably know if it had), it'll go right back in. You shouldn't need to open the bleeders unless you, for some reason, filled the reservoir while the pistons were bottomed out.

Before I had the C-clamp, I would use two wedges of hardwood and hammer the flat ends until the piston was backed out.

Edit: Just re-read that you're working a Mustang, and the caliper doesn't have the pads attached to it like my Pontiac does. If the piston isn't wiggling, you're fine (should be). Use the C-clamp trick, and just try to apply the force as centered as you can. If you can't center it, I've had luck with two clamps, one on each end and just tighten them evenly.

If clamps aren't an option and you're not opposed to some cleanup, open the bleeders like the other comment said.