Now I feel like I have a devotion to Moses, but I'm not sure if you can have a devotion to Old Testament figures.
Yes: one may have devotion to the Patriarchs and Prophets and other holy persons of the Old Testament. Their names are found in the Roman Martyrology, and Our Lord vindicated them by fulfilling their oracles and invoking their authority against the perfidious Pharisees.
An illustrious example: the Carmelites have always venerated the Prophet Elias as their Father Founder with a proper Mass and Office, as well as the Prophet Eliseus. St. Elias is the only
viator (for he has not died yet but was "taken up in a whirlwind of fire, in a chariot of fiery horses," Ecclus. ch. xlviii. 9) who receives official liturgical veneration in the Latin Occident. The Byzantines and other Oriental Christians also render homage to Elias, and probably the other mysterious
viator, Henoch, who was taken by God (Gen. ch. v., 24).
It is my understanding that one may offer stipends for Masses to be said in honor of any Saint named in the Martyrology, so I do not see why one cannot venerate the Prophets and Patriarchs named therein.