Presumably the laptop could be designed such that opening the case would separate some contacts in a circuit, which in turn would clear some important secret va

Presumably the laptop could be designed such that opening the case would separate some contacts in a circuit, which in turn would clear some important secret value from memory. If you have a separate authentication device, it could warn you that this had happened, to prevent the attack of someone opening the case to add a circuit which broadcasts your key presses, for example. This still only reduces the problem from keeping your laptop with you at all times to keeping your authentication device with you at all times, though.