You can use a C-clamp to squish the piston back into that caliper so the pad will fit.
Also, define "toast" in reference to the rotor? is it just grooved and ugly or is it a center finned rotor that has worn dangerously close to the fins on one face?
If it's just grooved/gouged and ugly you can damn sure put a C-clamp on that caliper to compress the piston and stick those new pads in there.
I can tell you for sure it won't be any worse than it was with a bare metal shoe plate grinding on the rotor face, might not be perfect but it could get her by for a while.
