Unless I'm missing something, there's currently no way to do this. Basically, it would be the same as the current Export button except it doesn't do anything with the images; with a big project, compressing images easily takes up a majority of the time it takes to export. When you have to re-export because of some tiny mistake or minor addition (which I have to do A LOT because of AJAX), it gets pretty annoying.
In short, this would be really convenient and I don't think it would take much work to add it in. Pwease?