No I did not have any more testings with columns, I focused more on meta-information is actually stored by dopus and explorer and did make some findings already:
What surely is of interest to you, who has a tagged image library:
DOpus does not necessarily store tags in the ntfs filesystem for image files (I tried png and jpg at least). I did some binary file comparison between equal images and different tags and additionally did not find ntfs information stored for these kind of files. But for files not carrying some kind of meta-data container right from the start, let's say simple txt-files, DO does uses (already mentioned?) ntfs alternate data streams to get the information saved.
Now in case you use some kind of raw-picture format DO probably not (yet) knows about, it may still use ntfs for your tags. I hope to find some more detail on what method DO uses for the various file types and image formats - just to be able to estimate what makes best sense to do, regarding usage of tags in the future for me.