An Airbus rep told me of instances of Green to Yellow transfer but it is not 'intentional'. It occurs in the brake shuttle valves (Green / Yellow system) perhaps when seals fail internally? Sorry can't remember the exact details now.
It is my understanding that this would only occur when the brakes are applied or pressure is sent to the shuttle valve from the Brake control unit. Normally not pressurised. Transfer would stop when brake pressure released.
Also, I'm not sure if the brake hydraulic fuses are before or after the shuttle valves. It's been a while. If they are before they will stop the flow when it gets beyond a normal rate.
In regards to the green system loss. If the warning is inhibited due to flight phase screening to 1500'. The plane does not fly around long below 1500' agl. (maybe a pilot type might tell us how long from takeoff to 1500') I'm tipping it's not long.
Your yellow system might get to an overheat but it still takes time. Depending of course on the amount of work it does in the PTU and other controls. It also depends on the rate of loss of fluid from the green system. It's generally going to keep tyring to make pressure until the reservoir is empty. A small leak will take some time to dissipate the fluid. The Engine pump will continue to make pressure though. The PTU will not come into play.