Having done some research myself I can't seem to come across any resources that explain how the LEDS work. I suspect the LEDs are something that are hardwired into the Ethernet port and are not controlled through software. For example take a look at this schematic of a RJ-45 Ethernet connector:
You can clearly see the two LEDs labelled YELLOW and GREEN are hardwired to the connector pins.
A hardware hack that might work although if your not confident with electronics do not try this. Is to de-solder the ACT LED and run a wire from each pad on the PCB from where the LED is connected and the solder the LED across the end of the two wires. I dubious as to whether that would work because the wires might introduce too much resistance.
There is a way of connecting an LED to the GPIO and it giving the same functionality as the ACT LED, you might find this thread useful. I think going the GPIO route is probably your best solution. You can thread wires from the GPIO through the case to an LED.