(SteveH) gurgled happily, sounding much like they
were saying:

I really fail to see any way in which a wind-back piston can be wound
back with a G-clamp, especially given that it doesn't sound like it's a
sliding-yoke single-piston design...

Here's a picture.

http://www.trevor-turner.co.uk/pages/Caliper_Small.html

Definitely designed for being wound in with the appropriate tool.

OK, so you can get a G-clamp onto the piston.

It's just down to "Will the clamp turn it rather than push it?" (Very
unlikely ime of turny-piston calipers) and "Is there anything solid
enough on t'other side...?"

http://www.trevor-turner.co.uk/pages...bly_small.html
Looks like there may well be.

Either way, it's still a bodge.