How about a lanyard that has a magnetic connector (like Apple charging ports). So it just a small amount of force to disconnect, but easy to stick back together if you forget it was there when you got up to visit the rest room.
Another thought, what about a little coin-sized watch battery device that does bluetooth low-energy. Press a button it locks your computer (or triple click wipes something, etc).
You should use the scenario of a pair of strong, well trained soldiers physically holding you or otherwise preventing you from performing actions. So anything requiring being aware that you're being raised then doing something like triple click might not be feasible.
I think a system would need to highly tend towards false positives, giving you a short ~5 second grace period to perform some positive challenge that things are OK.
And if this means that every day you end up accidentally having to reboot and start up Tor, well small price for physical security. But really, you should be far more focused on getting your online opsec right so you don't have to worry about thugs. If they're physically grabbing you it's very likely game over.
One countermeasure would be to find people and pay them anonymously to look like you. That is, proxy through their laptop, maybe even have them do some lightweight writing or chatting. Use their life details to leak things, like about weather or other local goings on. Essentially using them as a canary. If they get tackled, you know it's time to burn everything and hide.
A lanyard with a magnetic connector doesn't suffice. If the enemy cuts the cord, the magnetic connector won't release and the computer won't know anything has happened.
If the enemy cuts the cord, the loop of wire in the cord will be cut, and hence the computer will know something has happened.
(i.e. you have a lanyard with a magsafe-like connector with two pins. There's a resistance wire that runs inside the lanyard from the connector, up through the loop, and back to the connector. The computer checks that the resistance remains the same.)
If you want to get fancy, you can embed a RC network in the lanyard and have the computer sweep frequencies measuring reactance.
Another thought, what about a little coin-sized watch battery device that does bluetooth low-energy. Press a button it locks your computer (or triple click wipes something, etc).