Nightmare news Pete - you are correct it is almost impossible to cross thread something undoing it, I have thought of one long winded scenario but that certainly wouldnt apply in this circumstance.
How far is it to your mates Garage - one time I had a caliper fail whilst on a 300+ mile journey - I got the caliper off, fixed the pads in place by tie wraps then pumped the brakes to get the pistons pressed against them then tie wrapped the caliper out of the way under the wheel arch.
the car obviously pulled to the one side but careful driving, smooth gear changes and lots of engine braking got me home