How can I achieve the following:
In Thumbnail view, in a folder which contains images, every image which contains descriptive metadata (e.g. IPTC, JPG comment) should should have a small overlay icon on a edge (e.g. small information icon). This would be very helpful when adding descriptive data to the images. Is this already possible?
I don't think that's possible.
You could use Tiles mode and set Opus to display some of the EXIF fields next to the thumbnails. Opus does not currently support ITPC at all though and I'm not sure but I seem to remember something about the EXIF comment field not being in the list of supported fields either. (Maybe the Documents->Comments field works? Not sure though.)