However I don't think this should be a BIP as it doesn't need to be in the reference implementation,
a BIP is a "bitcoin" improvement protocol not "bitcoin core" improvement protocol. in other words they don't have to be in reference implementation (bitcoin core) to become a BIP. for example there are BIPs that aren't in it or even anywhere else for that matter. example: BIP178 which is a versioned WIF proposal which isn't used by any wallet. or BIP39 which is mnemonic code (generating a seed phrase part) is not used by bitcoin core.
I know but to standardize it (which is my plan) it needs to specified somehow.