I strongly suggest not to use any popups on proxified pages as it will irritate the visitor and he will be lost forever
And slowly you shall lose all the traffic
Imagine yourself as a visitor being irritated by popups?
The only time I would yse pop-ups is when you know you have a flood of overseas traffic coming. What Google and other companies call low quality traffic (from China and India etc.) But even most popup companies pay less for that sort of traffic too but you can earn a little on the volume.
For a proxy with loyal US/UK members I would avoid popups as much as possible and look in to other ways to make a return on proxied pages (such as AdBrite etc.)
Well you always have an option to reject that traffic
I always do that and so my visitor count stays low
But still its one's own choice if you ask me