Our process and why we are in need of this feature works like this:
Rough Layout spread - Place images, copy, and artwork
Often, images that were previously used on the left hand spread of the book will get moved to the right hand spread. 99% of the time that means the image needs flipped to face the center of the page. Sometimes it's ok to flip the image if the content is symmetrical or non directional, but when the product cannot be flipped it needs reshot. However, as a placeholder until the new image has been shot, the image remains flipped until its replaced. So one reason we need the feature is to be able to quickly see which images were flipped by the layout team (usually a note is made but not always).
Proof Each Spread
Once all the images have been shot and the new ones place, copy written etc, the spreads are ready to be proofed. Part of our proofing process is to make sure when the new image was placed that it was flipped back. It seems like an obvious part of the process but they do get missed. This is the second place we would need the feature.
Having the feature in both the resource manager and on the image itself would be useful. Since the resource manager is not part of the toolbar, you do not always have it open. Therefore when clicking on images it is another step to go and open the resource manager just to see if the image is flipped or not. In this case an indicator on the image box itself would be useful. However, it would be useful to have it in the resource manager as a sort of preflight check like you mentioned. One stop shop to view all the files at once. If the field was able to be sorted that would be even more useful since you could have all of the flipped images grouped together.
Side note: Unless I am mistaken the resource manager is a floating panel only and not yet able to be pinned to the sidebar - which would also be a handy feature to have)