Yeah, unfortunately there's not currently a good option for this. There have been a couple of attempts to build power meters into shoes in the past, but those didn't really go anywhere. If you haven't seen it already,
this is a good overview of the market. There is a company that's got an SPD-based power-meter pedal in the works, which I believe is a first.
I think the problem is that the power meter needs to be calibrated very precisely to the component it is on/in, so moving the power-meter mechanism around just isn't in the cards.