We made some good progress internally towards implementing this, but the refactoring for Stripe Elements and SCA has taken 2-3 times the expected amount of development and unfortunately we are not able to develop this under the Make It Happen funding. If additional funding becomes available we would be interested in developing this.
In Administer->CiviContribute->Stripe Settings set the "Country" setting to a valid Stripe country per the description. Leaving this blank disables the paymentRequest button.
For a supported device the "card" element will be replaced with a "Pay now" button:
It would be nice to hide the "contribute" button in the future but it will trigger the same action as clicking on the "Pay now" button. Both submit the form automatically on success.
I'm not sure whether it will work properly with recurring payments etc. yet as they work in a slightly different way.