I think a general "append_version" tag would be fairly worthless. I'm not familiar on which they're updating, but if they're all or mostly all going to be receiving some append update, then the tag would be fairly worthless. On paper it might seem like a great idea, as you can just put that on there, but in practice it'd be fairly worthless. What if only one character is appended and the other characters are not? What value does the append_version tag offer then? Nothing, because all it says is that there is a character who is the append version, but it does absolutely nothing to tell you outside of that, is it Hatsune Miku who is appended? Is it Kagame Rin? All it says is that one character is appended, and fails to distinguish who, so if you perform a "character append_version" search, you'll end up with every image in which an append character appears and the other character appears, which may well be filled with images in which the character you're looking for is not append version.
tl;dr: An "append version" tag will be worthless if every character gets an append version, as the tag fails to distinguish which character appears appended. In images in which one character is appended and has other characters present, you'll end up with noisy results because the character you're looking for may not be the append version.