Ah. In that case, you'll have to use LOW = WAKE if you use external interrupts.
You may be able to reduce or eliminate the sleeping power consumption by enabling the pullup resistor on the input (slave) side. When the master sinks (turns the output low) to wake the slave, current would flow. When the master sources to sleep the slave, both sides would be at the same potential so no current would flow. I think I have that correct.
You can still use pin-change interrupts to get what you need. Unfortunately, there's no support for pin-change interrupts in the Arduino libraries. They have to be enabled by directly manipulating registers.