If you can self service the battery, then you can obviously remove it, and with no source of power there should be no real risk of electrocution
I would be surprised if the manufacturers don't build something like that in already, just because it makes the car safer to assemble. (Wiring up a high-current battery pack to a bunch of electronics can be... fun... if there's not a cutoff close to the battery. You end up with a pretty good-sized arc in some cases even with the other electronics 'off'.)
The Prius has an emergency disconnect under the hood that can be accessed by emergency personnel / firefighters, although I believe it's destructive to operate. (It's a "cut here in emergency" sort of thing, at least on the older ones.)
I could see having a standardized service disconnect switch, or at least standardized signage or switch appearance, on electric/hybrid cars as a safety thing, not just for roadside emergencies but also for maintenance.
There are two batteries: 1) normal 12V battery (in the trunk under the floor I believe) 2) the large 'main' battery which
The 12V is absolutely user replaceable. Wonderfully, they even provide jump lugs under the hood in normal locations. The main battery is under the rear seat and the center console between the passenger and drive sides seats. I haven't messed with that one...not going to bother until the warranty is gone.