The problem I’ve seen with foot-worn power meters is accuracy. I used a Stryd a couple of years ago and it was horrible. Maybe it’s improved, but I’m skeptical because it’s an algorithm rather than a direct measurement of power.
But if we could get accurate measures of power for weight-bearing activities, I would switch to it immediately.
You would measure AeT the same way. Find a constant power output (ideally on a treadmill so that you know the output is constant) that creates a heart rate drift of less than 5% over the course of 60 minutes. Then when training, keep your AeC sessions below that power level (and ignore heart rate).
We would use power if we could, but from what I’ve seen so far, it’s grossly inaccurate. I would love to find a device that proves me wrong.