Yes of course, I should have thought of that. It's been discussed before.
Are you dead set on a passive solution? You might consider a Raspberry Pi (or similar small computing device) with a microphone, which is programmed to listen for the exact tone that your washer emits at the end of the cycle. That could call an endpoint in HE to trip a virtual switch. Obviously not as simple as a power sensing outlet, but one Raspberry Pi could be programmed to listen to both your washer and dryer's end of cycle tones and kill two birds with one stone.
If your washer doesn't have an end of cycle tone, you could listen for some other sound, like the sound of the door/lid lock solenoid releasing, etc.
