He appears in mizuki_hitoshi's comics and as you can see by searching for tani_takeshi mizuki_hitoshi, they include the tani_takeshi artist tag, which isn't a good solution since he didn't draw those images.
I though of tani_takeshi_(character) or something like that for a name. (it's not as convenient as with zun)
Edit: Vice versa for mizuki_hitoshi in tani_takeshi's comic (post #1063233), but I guess those can just be tagged tatara_kogasa. (but the information that it's mizuki_hitoshi is "stored" in the artist tag)
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