I hate to be pessimistic, but to be honest, I think that leaving it as OPT OUT is actually the better way. I don't believe that privacy concerns are really all that important. While we'd like to think that, the truth is that privacy is basically gone now, and clinging to principles is idealistic and naive.
I'm not saying that I'm not idealistic and naive. I have a hard time with the issue myself.
What does also work is to simply use an autoupdater that includes a unique ID. Many many methods to do it.
A "Tip of the Day" screen can pull in an updated version.
A news feed can pull in RSS/XML and update a ticker in the UI.
An info ticker can do the same.
Offering dynamic discounts through a get/post to your site works well to boost sales AND accomplishes the same thing. Tagging the discount with a custom purchase code lets you do complete tracking with many more benefits.
etc. etc. etc.