Negative charge pump problem
I'm going to solve this by placing a 30V zener diode across the main filter capacitor, between Vgnd and Vout, to keep it at 30V maximum. In case more current is drawn from the auxiliary negative supply than from the main positive supply and the voltage starts to rise as described in the question, the zener diode will kick in and compensate by equalizing the difference in currents. I have just tested it on a breadboard and it seems to work well.