I have thought of this method (or something similar, using a global variable). However, I don't think this method would work here. My Run callback is something like this:

foreach(@list){
  DoLongBlockingThing($_);
}
[download]

so, I would need the cancel button to immediately break the LongBlockingThing() routine and not have to wait for the loop to come around and check the presence of the temp file (or variable).