Oh, I see
@juvela mentioned me a while back... sorry, I've been out to lunch :lol:.
Anyway, I think you're already on the right track, lifting up the flakes that appear ready to come away. Tape is a good idea. You might also try that gummy stickum stuff they sell for college kids putting up posters on the dorm room wall... know what I mean? It's sticky, but not very sticky.
With no desire to criticize anyone, I have to say (a) avoid heating the leather in any way. Heat may soften the paint, but it will cook the leather, and given that the leather is already 50+ years old, it's already past its expiration date. Also, (b) avoid solvents. Anything that dissolves the paint is going to do serious damage to the leather.
One thing I would
consider is water. To be clear, I think it's a bad idea, but (that said) here's my thinking: water usually weakens the bond between paint and whatever it's on, especially if the water causes the thing to swell up a little, stretching the paint, which would then probably flake off more easily. But you'll do more damage to the leather, so I can't recommend it. If you do try water, the important thing is to put water into the leather without leeching anything out of the leather. Whatever soluble material is in the leather, you want it to stay there. You could spray the saddle with a little water, let it soak in, and spray it with a little more, and repeat, and then put it in a plastic bag for an hour or two so the water permeates all the leather, then try the xacto knife again, see if the paint peels off a little easier. If it doesn't seem to work, let it dry again. After it's been wet and dried again, I'm sure the paint will flake off a little more easily. But you will have artificially aged the leather, not for its benefit.
Good luck!