it’s amazing – I had nearly the same idea last november. Since then, my mooshimeter is switched off completely if not used. :-)
The difference between our soulutions is that mine not only preserves cat III compliance, it also doesn’t void warranty, as it doesn’t require any hardware modification. I used a snippet of thin double-sided PCB material as a carrier for the reed contact, which can then be “plugged” between battery and the corresponding contact, so it’s a completely removable “addon”.