but has been done, as this classic caving tale relates!
If you are using a standard UK frog rig then there is a way of removing your harness on rope while ascending, next to last resort perhaps and takes a little time, and much more difficult when cold, wet and exhausted than in a dry warm practice situation. Better than dying though. Takes around five minutes in comfortable surroundings. A couple of spare krabs make it a lot easier.
First, get rid of all excess gear, tackle bags etc, clip to belay.
Next fashion a temporary harness from the rope below you, a figure of eight and a y-hang below it essentially, step into into all three loops and then move the knots up the rope until they are just below the croll, try and keep them snug around waist and legs as much as you can, get as much slack out of the three loops as you can. Clip your Stop into your cows tails belay knot (i.e. the middle one) and through both knots, which is where the extra krab or two are handy. Install your Stop on the rope just below your chest jammer.
Now you can take your harness off.
Very hard in practice because your harness is weighted and your new belay point will inevitably be as much as a foot or so below the old one - you might even have to cut it off BUT if you are able to climb up a foot or two and clip your new harness directly into a belay - using your short cows tail or a couple of krabs, or whatever, and thus unweighting your "old" harness it becomes a viable escape method.
When we worked this out twenty odd years ago our starting point was "What could you do with a standard rig to replace your harness mid rope" - i.e. no spare kit.
Why did we bother? One of the lads I was caving with at the time had a problem, he was basically using an off the shelf setup but his pride and joy was his home sewn harness. Near the top of a pitch it decided to unsew itself, fortunately he had hold of the traverse at the pitch head at the time and he was able to climb out, and wander off shopping for a new one. We decided to try and work out if you could improvise a harness should you realise yours had or was about to fail, and a jammer stuck under a knot was one of the things we thought about.
We eventually decided on balance it was much better to make sure you don't start shoving jammers hard into knots. If anyone can improve on it I'd love to hear about it.