That last approach, using Lightning address comments, is vulnerable to spoofing though. I could send a Lightning address payment with a comment but in the comment make it seem like it was from someone else. Maybe SN withdraw that uses Lightning Address could be modified to add a comment field, and then if it was a withdraw to another SN account, SN could then (for the notification for the recipient) be able to add the sender's nym since it would be coming from the SN node and thus has account info for both sender and recipient.