You can have an immutable centralized/federated system without using "crypto" though "proof-of-work", there are databases out there that provide immutable storage that don't require this overhead.
Providing api access to a signed immutable database makes sense... but I'm not sure how much sense utilizing existing popular cryptocurrencies would make (e.g. bitcoin, etherium)
Providing api access to a signed immutable database makes sense... but I'm not sure how much sense utilizing existing popular cryptocurrencies would make (e.g. bitcoin, etherium)