I guess you could try modifying your development board to test that, but keep in mind that the Rabbit I/O pins are 3V3 and not 5V. You'll want to use a gate to control the board's Vcc based on presence of the USB cable. Additionally, if you go this route, you won't be able to use the FT232 interface as a serial console (serial port A) when your program is in "run mode".
You'll want to test it out, but I think you'll need to manually reset the board when you disconnect the USB cable. My recollection is that the Rabbit only checks the SMODE pins at startup.