I don’t use Stripe and can’t say without a lot more info, but I’d probably recommend triggering the behavior you want in some custom JS, which you can load with ->addJS() within the boot() method of an extension. I say so just because there’s a lot of async JS happening on load in the payment modules, and you probably want to skip all of that, as well as avoid any changes to files in payregister, which are bound to get overwritten the next time the extension updates.
