Hi Geert,
Direct user interaction with image windows is not possible from scripts, since script dialogs are modal. PixInsight 1.8 introduces the new ImageView component:
http://www.pixinsight.com/developer/pcl/doc/html/classpcl_1_1ImageView.htmlImageView allows embedding an image browser, with the same functionality as an image window, in any control. Unfortunately, ImageView is only available to PCL for now. A PJSR implementation is still not available, but it will be soon after the 1.8.0 release.
If the tasks that you're going to implement are relatively simple, a possible solution may be a ScrollBox to browse a Bitmap representation of the image. Like any Control, a ScrollBox can receive and handle mouse events.