The power connectors will be on the far side of the rack from the service side so shouldn’t be a problem for humans touching the third rail so to speak.
With that sort of voltage you should be able to use a capacitive or inductive sensor to activate a relay.
Obviously the contact point in the sled will need to be disconnected while not grounded, as the top level suggested, or shorted to ground across a large resistor. It’s not the amperage that’s going to cause problems it’s the 800 volts, backed by the amperage. Small enough capacitance and large enough ground load should let you tap in without frying anything including air.
Wouldn’t the biggest problem will be loading sleds into a hot rack, with all of the sleds powered down? Which probably you shouldn’t do. The parallel loads on a rack that’s half on should sink a lot of the potential.
With that sort of voltage you should be able to use a capacitive or inductive sensor to activate a relay.