replaces the xpub decryption key in BIP-138 with a signature
I think this is a major footgun, I really doubt your signature will be deterministic between signing devices/versions
BIP138 do not prevent you to use an xpub like from m/138'/ derivation path and use it to encrypt, this seems have same properties you are looking for
and tbh I’m not sure to get where is it operationaly better if you have a good xpub hygiene