Before stumbling into Mikrotik, I looked into the “redirect” feature of squid to do advertising on my LINUX-hotspot. For my taste, it is more user-friendly than popups, because the original page is presented after using the “Refresh Button” of the browser, after presentation of the ad. And the usage of the “Refresh-Buttton” can be timed, so that the user is forced to have the ad a minimum period of time in his browser window.
But I was stuck because of my “quick and dirty” implementation, when just by chance the ad showed up in another window, without refresh 
Keeping track of open windows, focus or similar should do the trick, I guess.
But, because I am just a very beginner in JS, for you guys it should not be a big challenge, right ?