I should clarify that Infinite Scroll does offer a callback for just this purpose, and I have used it successfully with some other scripts (a slider, a lightbox…). I just don’t know how to reinitialize MapPress since it’s a plugin and not a simple script that I loaded myself.
Also, this doesn’t happen with all plugins. For example, I have a Facebook plugin that uses a shortcode to display my facebook feed. And that loads fine on all “pages” of the infinite scroll.