I think your suggestion of using a pressure sensor is a great mechanism. Then, as you say, you need to be able to restore the keys (stored offsite) so you can

I think your suggestion of using a pressure sensor is a great mechanism. Then, as you say, you need to be able to restore the keys (stored offsite) so you can do maintenance. This is an engineering problem: how secure do you want to be; how many mechanisms do you need to achieve that; does this come in under the cost budget. Your pressure-sensor solution alone is probably good enough for a large number of applications already. But this is not a research problem: we don't have to solve homomorphic encryption!