I doubt you have to be in the engine bay to turn off that system. Unless there are special satellite comm systems on the engine nacelles, that data is probably repackaged by the jet’s comm package and pushed out the satcomm antenna with everything else. And even if they *are* transmitting from their own antennas, they have breakers, too. Otherwise, a malfunctioning telemetry system could cause a fire itself if not disabled on fault.
Every electrical system on a jet is monitored and pretty much has a off switch in the cockpit, as I understand it. The reason being, is that any system can malfunction (even safety systems), so you have to have a breaker to switch off power, in case that system starts misbehaving. You wouldn’t want your flight recorder to crash your very expensive jet, killing your expnesively trained crew & paying customers in the name of safety!
Reasonable redundancy can only take you so far in the safety game. There are systems that if they fail, you won’t be flying anyway, I suppose some of those probably don’t have cockpit breakers, of if they do, they won’t be cycled off long.
A toilet almost bought down a cargo bird a few years ago. The ground crew put too many of the detergent balls into the toilet (makes the water blue and kills smell, etc). The balls foamed up too much and the foam mixture overflowed the holding tank and the water/foam mixture dripped into the avionics bay and started a fire. The crew was able to make an emergency landing.
Makes sense to me. Unfortunately, it all breaks down if you have a flight crew that’s doing the nefarious activity.