Trains from the Charing cross branch can go round the Kennington loop and head north, no need to remove passengers, if they don't get off the so be it. trains from bank would have to tip out and have staff check they were empty and reverse via the Kennington siding, this is unsustainable with the current service levels, regardless of the fact that one difficult sleeper who will not leave the train would end up with a service suspension.
The reason they have to be tipped out is because the driver has to walk through the train when reversing via the siding but can stay in the cab when going round the loop.
It used to be that the reason passengers could be taken round the Kennington loop was because it was a colour signal move. Indeed, many years ago this was a booked passenger move when CX reversers non-stopped the SB platform when it was closed due to the IMR going bang (or whatever!). Passengers for stations beyond Kennington got off at Waterloo and waited for the next Morden train.
Drivers were always taught that passengers shouldn't be taken past shunt signals unless the route was secured because there wasn't the same amount of safety involved (point detection, locking, or whatever, I can't remember now). However, I think that over the years that may have improved and that on the running lines a shunt move ended up just as safe as a colour-light move.
It certainly didn't bother LU when trains became driver only and the Drivers were then told just to announce "all change" then close the doors before going into a siding instead of doing a platform detrainment. This, of course, led to more passengers ending up on trains in the siding and the potential safety risk to drivers from stroppy passengers. Some drivers would still detrain anyway (sometimes I did, sometimes I didn't). I think that eventually this practice was stopped, possibly due to at least one passenger fatality) and usual detrainment methods were applied when reversing in a siding. Even in the loop where up to three trains could (then) be held, it was not unusual to hear the slamming of doors as a passenger would start walking through the train after it had been held for a while at one of the signals there.