Instrumentation Amplifier REF pin maximum voltage
They do not list anything on the datasheet about the REF pin, although it clearly lacks the 'overvoltage protection' shown on the (other) input pins. 





Yes, you should keep it within the supply rails, or at most a few hundred mV beyond the rails. Since you need to drive it with a low impedance it would be best to feed it with a buffer that is also powered by the same supplies. 

That's for protection- for operation see the datasheet figures 3-6 and think of the supplies as being relative to REF. You need to prevent the internal nodes at the outputs of A1 and A2 from saturating as well as the output of A3, and to keep the inputs of A1, A2, A3 within their common mode range- so the region of proper operation is a different 3D volume (based on the voltages on the inputs and REF on each axis) for each pair of power supply voltages.