Load iframe on slide enter

Up until now I have used slidify with the ioslides theme. As that's not supported, I'd like to move to a different framework. Xaringan seems very nice, but one feature which is essential for my purpose is being able to integrate iframes in a slide. The issue is that these should only be loaded whenever the slide is visible, and not before (it's Mentimeter questions, and all goes haywire if all are loaded when the first slide is shown). Is this possible, and if so, how?

Thanks for any insights!