Wouldn't it be easier to just stick a copy of the barcode somewhere onto the phone's case?

You can check out books even if the battery dies.

If this end result must be attained:

https://iliana.fyi/blog/ios-wallet-library-card/pass.png

can be achieved by displaying simple PNG image.

The point of applications that can store cards is that they enable use with wireless payment terminals. If I couldn't tap my phone to pay with credit cards, I wouldn't have a reason to put them into the phone.