Bingo.

Noninvasive, with any sort of accuracy that can be used for anything, is most likely impossible.

So back to "invasive"... what can we do... embedding a device under your skin, right around where your watch goes, that can be powered and communicate with the watch. While also making sure the device is not rejected by the body and stays in place.. AND has no other adverse side-effects. I think this is where it'll ultimately end up for the people who want it. sub 10 min procedure to put it in and should last for quite some time (maybe dissolves after 5 years for upgrades?)