Would it be hard to change the display of Private messages so that the sender can tell if the recipient has read the message? Not that I want to see delivery confirmation or notification or such (perish the thought!) but how about a simple indication at the top right corner of the sent message, similar to the way that Discourse indicates if a post on a public forum was sent via the web interface or email?
I just sent a user a PM, and not long after, I think. “Damn, I forgot something”. If I could see that the user(s) haven’t read my message, then I’d just edit my original, otherwise I’d reply to my original.