Can't do it.

It only works with stock Android and I lug around a second phone specifically for this.

They're using hardware security features that just so not function with aftermarket images. I can't remember the details, but working around it would require breaking the hardware security feature. They're trying to guarantee credit card companies that the cards are always being protected by their security approved code and it's not being undermined by someone having a weaker O/S. Of course, this also prevents people from using it when they have a stronger O/S.

Ironically, Google Pay works just fine on devices that are long past end of life and haven't gotten any security patches in years. Let's hope they don't change that so I can keep using this cheap old phone for credit card taps while my actually secure one can hold my personal data.