It'd be OK if you added it in the contract. "Deliverable will degrade in visual appearance through a 10% daily reduction in opacity if payment is not made on the schedule and according to the terms described in section X.Y below. Full payment including any applicable late fees or interest will restore the site to full functionality and visual appearance and will remove the opacity reduction code."

At the very least it'd spark an interesting discussion with the client that might allow you to reach another measure which would satisfy both parties.