alt Hacker NewsI've considered building some kind of anonymous email or sms system that seems similar to your idea. My idea was inspired by Craigslist's email proxy system which is pretty much the same thing. When you send an email to a seller, you get a generated email to send to, then behind the scenes, CL takes the email and sends it to the lister with a generated "From" email.
ETA: But to answer your question...... I'm not sure if building something like that has enough applications to be worth building :)
I think a low-tech solution might work. If we just require that the emailer put their HN username into the subject line, I could quickly look at the profile and see if they have at least 500 Karma or something before I even bother responding.
I suppose in my profile I could put a template like "HN - <username> - Question" and explain that if they don't follow that template I will ignore it.