Aside from when gardening tags like casual which should never be used on one-off original characters, is anyone actually going to make use of a tag to cover all repeat original characters?
Because you can already make tags for unnamed original characters. But it can be very difficult, unless the artist confirms it, to tell whether a character is a repeat character or someone completely different drawn in a way that they look basically indistinguishable. Take an artist like matsunaga kouyou for instance. Back when he had only produced a couple of hundred images I tried to sort them out into distinct characters - there were definitely groups of images that seem to be the same character. But I found one character rapidly blended into the next and then into the next. Now he has over a thousand images, and the more obvious variations have actually reduced over time, making it even harder to distinguish - yet he still produces groups of images that very much look like the same character/s.
Creating an overarching tag for this type of character does not resolve this sort of problem - if anything it accentuates it as it creates more pressure to determine whether a given image contains a repeat character or not, whereas at the moment it can just be left alone without causing any problems.
If literally all you want is a list of original characters then the existing wiki (/wiki_pages/23837) works far better than creating a tag like this would.