core devs are being too hard-assed about it in my opinion 
we don't sign a message from an address. we sign a message using the private key just like we sign anything else including transactions with that private key. and the standard is already there (add a fixed string at the beginning -> double SHA256 hash -> sign -> encode with base64 with a recid).
that is what we have been doing all this time with legacy addresses and that is what Electrum continues to do with all types of addresses.
we don't sign a message from an address. we sign a message using the private key just like we sign anything else including transactions with that private key. and the standard is already there (add a fixed string at the beginning -> double SHA256 hash -> sign -> encode with base64 with a recid).
that is what we have been doing all this time with legacy addresses and that is what Electrum continues to do with all types of addresses.
I don't have the technical knowledge to confirm what you are saying, but I believe you are correct.
But I believe it is important that core validates that, so all wallets would simple sign those messages.
For now, electrum signature cannot be verified in most of the softwares....