The ban is not on encryption per se (which, as you point out, could be confused with cryptography in general), but on attempting to obscure the content of messages. Obscuring message content is by no means a function of digital signatures.

For ECDSA you can’t provide a signature isn’t arbitrary data. The nonce is by definition not known, so it contains enciphered information.

That's true but not relevant here because the intent of the signatures is not to conceal the nonces.

I think you mean it can't be proven not to have 8 bytes of enciphered information.

But anyway, you can use a hash function to generate the 8 bytes of nonce.

You can’t prove you did without revealing the nonce, and therefor the private key.

You could reveal it once the transaction is used. If anyone really doubts your 8 bytes.