Kai,
I couldn't quite understand how your circuit worked initially as the gate would be driven negative - that is until I read the datasheet on the mosfet you proposed. I'd never known of such devices before! Your comments regarding zeners are noted - MicroSim didn't seems to take these effects into account as the simulations I did today seem to work how I anticipated - the circuit I was simulating was similar to Jose's (this was the initial concept I had in mind). The only issue I have with using the DN2540 device is that it may be a little difficult to get in Australia, so I'll have to do some research. Nevertheless, it is a good application for this product!