Thanks for such a comprehensive response! I've read it a few times, have checked JohnJohnJohn's post, and have read the section in the manual on usertracks, but unfortunately I don't understand what you're saying

I'm sure it's my fault... is there a step-by-step guide or tutorial I could have a look at that might help me understand?
I'm not sure why the long and difficult to follow explanation from Charlie...
In the end, I think he is just saying "+1".
I think JohnJohnJohn's solution is exactly what we are all asking, for... the ability to isolate and freeze bars from tracks and then fill the unfrozen gaps with a regenerated performance.