Airplane wings are designed to stall in particular areas first. I guess these foils are rather small for that.
If the horizontal surface cavitated, would the problem be limited to that area? Would it be possible for that to immediately ventilate the rest of the foil?
As Sam mentioned, the cavitation will start small on a small section of the foil where it is creating the most lift (vacuum). If speed continues to grow, the cavitation will spread. In this case, the cavitation will limit the lift of the foil and start to slow the boat down as it tries to maintain foiling (or the hull touches the water). It probably wouldn't be very dramatic.
