I just tried the Maildrop Service. Is it supposed to attach the mail file? Sure would be helpful if you could double click it, and open it in Notes.
If you want to accomplish what you're asking for, you'd have to have the email you want open in full view, save the email, then create a new email to your omnifocus maildrop address and attach the mail file in that email. I'm not sure if this would allow you to double click that mail file from omnifocus and open it in notes or not. You'd have to test it.
Forwarding is a quicker process, and would give you the info you need to respond to an email, but you'd have to open notes and find that email to respond. I'm sure someone with scripting experience could figure out a work around.