Want WP post links to open in external site's iFrame

I would like links for individual blog posts to open in an iFrame on an external Web site. What is the best way to do that? Dynamically change the source URL of the destination iFrame? Some kind of redirect? I’d like to make this happen automatically so the author can publicize individual posts as opposed to a static blog link. I have an idea in my head how this might theoretically work, but I’m stuck on the execution. The destination site is not PHP and I do not have access to the headers. Hence the iFrame solution. Any help would be very much appreciated. Thanks!