I suppose I could release a new version with a new warning selection to fire a document event. At that point, you could have a custom JS listen for the event and do whatever you wanted. I could make the timer accessible, so your JS could display the timer too.
But the advantage of you're solution is that it's more flexible...
I did not realize the modal was changed to bootstrap in this plugin a while back. I think I need to check all our sites to see how session keeper is doing there. And maybe fix it with this new JS version.
We also use language string for the translation. Will the translations be visible in the JS triggered popup?
Great work. Hope to be able to test / implement this soon.