If the folklore is the rationale for the custom, the food to eat on the Ox day during the doyo could be 'udon' (Japanese wheat noodle) or 'udo' (Aralia cordata), since every food other than eel is possible when it has the Japanese syllable "u" at the head of its name.
この民間伝承を根拠とするならば、土用の丑の日に食べるものは鰻でなく「う」の付くものでいいのだから、うどんでもうどでもいいことになる。