Murphy,
While I agree that you should never heat a plastic tank with anything which can attain temperatures in excess of the working temperature specified for the tank, I disagree about the suitability for use as a reaction vessel.

With oil preheating performed before adding to the plastic processor, the plastic processor would be no more of a fire hazard than a steel tank.

Where the hazard comes in, is where people use high wattage, poorly designed electric heating elements to heat vegetable oil in the plastic reaction vessel. Even with a thermostat, the element temperatue can exceed the safe working temperature of the tank, causing it to distort and leak, potentially exposing the heating element and causing ignition temperature to be reached quickly.

A sitable heating element should not be able to exceed the safe working temperature of the tank and never reach ignition temperature for biodiesel. Even with a high wattage element, this is possible to achieve. It just needs to be designed for the purpose.
Features of such a design may include a large diameter tube enclosing the element, with a heat transfer fluid between, a 2 stage thermostat, which prevents the heat transfer medium exceeding the maximum working temperature and a fail safe temperature fuse which cuts off the heat if the design temperature is exceeded for more than 20 seconds (more or less).

Such a heating element could be safely used with a plastic tank.

Regards,
Tony