how about not using a rj45 jack, and instead, provide the user with some interchangeable cables, one end is special, and goes to your ecu, the other end is rj45, for connecting to a laptop. the other cable would connect ecu to display, providing the power you need.
alternately, how about an rj45 jack and a display jack
or you could have the firmware test the type of connection when it senses a disconnect and reconnect ... power up the data lines first and try to get an ethernet link ... if it doesn't link up, then turn on the power lines ... you could have the firmware monitor current demands on the power lines, and say, if the power is more that 100mA, there might be a short and shut them down.