They would not. Best case scenario is that you can trick Google into thinking your device/OS is legit. It's of no benefit to them to in any way compromise the security of their highly sensitive payments app.
Here's an example and why I'm not on a custom ROM. My job has "restaurant credit" as one of the benefits. That's 200 euros a month in a card that can only be used through Google/Apple pay.
So either I skip the money or I need a second phone with me all the time.
That's very short sighted of you. I value my privacy, but 200 euros a month for using a custom ROM it's a very expensive price.
My company values me, but it's a large company and making exceptions like this is an accountant s nightmare, specially for a benefit that's not even part of my contract.
Here's the summary for the wikipedia article you mentioned in your comment:
A ZIP Code is a system of postal codes used by the United States Postal Service (USPS). Introduced on July 1, 1963, the basic format consisted of five digits. In 1983, an extended ZIP+4 code was introduced; it included the five digits of the ZIP Code, followed by a hyphen and four digits that designated a more specific location. The term ZIP is an acronym for Zone Improvement Plan; it was chosen to suggest that the mail travels more efficiently and quickly (zipping along) when senders use the code in the postal address. ZIP Code is a trademark and ZIP+4 is a registered trademark of the United States Postal Service, which also registered ZIP Code as a service mark until 1997.
Privacy isn't a binary choice. Obviously I would love for there to be an alternative to Google Pay but no such service exists.
The most private form of payment is to use cash but that's just hugely inconvenient in 2024. Sometimes it would be nice to be able to pay from my phone when I don't have my wallet on me but custom ROMs don't allow it.