Thanks for your response. I've been requested in the Github BIP 322 thread to forward this to the mailing list. I've taken care of it.
It is possible, but tedious. If you can sign some message with a single key, then you can do it with multiple keys, just by providing N signatures. Unless you deploy N-of-N multisig, wrapped in Taproot address, then a single signature is sufficient.
If you have any information or bitcointalk post about it, please let me know. Currently, I haven't found any way to sign a message with a n-of-m multi-sig (n != m).