Oh, this is clever. https://delvingbitcoin.org/t/chain-code-delegation-private-access-control-for-bitcoin-keys/1837

I feel like something similar could be achieved using standard multisig setup but picking random keypaths for each change address (and keeping track of keypath per change output…which could be easily done with a deterministic way just like the original idea behind keypaths).

Could a custodian see every transaction? Yeah, but they’d have a few hundred million or billion keys to check…which isn’t as good on a privacy scale as the scalar tweak, I presume.