"There is a one part epoxy available, that is heat cured, but
it requires 2-300C for 15 seconds, which I think will melt the kapton substrate."
?
A Kapton substrate will easilly survive 300C for 15 seconds. There are grades that will work at 400.? If it is amber / brown translucent it is probably Kapton. If colourless or opaque yellow it may be polyester. Test the edge with a soldering iron.