I was thinking something like a high input impedance comparator with a clamping diode to substitute the transistor...
Edit: I just saw you edit now, and a MOSFET could also be used, thats right, with a clamping diode on the base. I've got high-voltage bridge-rectifiers, so I would probably place the clamp between the bridge and the MOSFET.
The other day I asked a question about driving a MOSFET gate from a PIC, and I ended up using a 100ohms series resistor - would this be nessasary in this case too?
More edit: Actually the MOSFET is not a good idea, since it has gate capacitance, a comparator would probably be better, or am I wrong?