What would an umbrella tag resolve? To me it feels like you already answered your own question by acknowledging that fine art parody exists, therefore, posts with the copytag without that would just be the artwork itself, unless you get some tacky situation where you have both a parody and the normal artwork which an umbrella tag wouldn't make any real difference in that case.
A new general tag when it's just the artwork itself, which isn't an umbrella tag, but is what you're suggesting here, would work if you wanted to have a tag to add to posts where the original artwork is present unedited but it's utility on an anime-focused website seems dubious at best, in my opinion, though I guess it is a taggable concept.