This would be to hard of a change to implement. Each stall would need to be able to store it's state and handle resets. Stalls are built as very thin clients with no storage besides dairy. There are many different ways to handle consent on a farm basis and now individual bull basis.
The ability to make each individual stall either consent or no consent instead of limiting it to a sim setting. Even if there is a HUD update most Bulls would still be hesitant without asking first. If a stall was in a clearly marked no-consent area there would be no need for that and would make RP more seamless. At the same time it would allow for Cows who want that power to deny to milk at the same sim in peace.