Just to be pedantic but I still don't get your point. The simple fact is that calibre does not support custom columns being returned from a metadata plugin. Which Kovid had already answered for you twice on this thread. The natural conclusion is that calibre would require changing to be able to support them. How many more ways can it be said - there is no "when" question here that I can see?
Sorry, must be tired.
Good luck with working on this and hope you can see it through, so one day my own metadata download plugins can make use of it...