Correct me if I'm wrong but could you not just use the built in 'Request Read Reciept' which is in Misselanceous (Sorry about spelling) Options when you send a PM.
I could and do but users usually just deny it. I could just turn off their ability to deny it but then they couldn't deny it to each other either and I don't want to do that. I just don't want them to be able to deny it from admins.