I'm not sure that would be a conclusive test, though. Before hydrogen production you have n solid mass + 0 gaseous mass. After, you have n - m solid mass + m gaseous mass, and after drilling a hole, you are only back to n - m solid mass and a lower density. Granted, m is very small, but couldn't the density change possibly make a difference?
As far as I'm concerned, the hydrogen production is plausible enough that I'm willing to take that as an explanation even without testing. As long as it's an established fact that hydrogen is a product of battery operation, which AFAIK it is, then pressurization of the canister seems an obvious result.