From rxc in comments:
The engineer’s solution:
Everyone who participates in the regulated activity needs to purchase a simple push-button device which controls a separate panel with a red light and a green light. During the regulated activity, each participant must hold down the button on the controller that illuminates the green light, which shall remain visible to the other party(ies) at all times. If, at any time, a participant releases this button, the green light goes off, and a red light comes on. Optionally, a siren could start to sound, to summon outside assistance.
These days, with Wifi and Bluetooth, I bet you could make the controller wireless and small enough to easily hold in one hand, leaving the other hand free for other activites…
I bet the Chinese could get something set up in a few weeks, and have it in stores by Xmas!
Failure to use such a device is cause to dismiss later allegations that consent was not given.
Batteries not included.