Hey, Knuckles got his own game, so why not Tails?
Go back through the ROM with the sprite palette again to see if any other characters have unused frames.
In the ROM, there appear to be two unused frames for Witchcart, suggesting that the animation for her defeat went through some changes. The unused frames seem a bit more exaggerated, and contrast with the used frames she has for the introduction and boss encounter.
Hold Up + 2 at the title screen and press Start. The menu title is a bit misleading, though, as it "only" contains a sound test and level select.