I don't know what kind of throttle you have, so it's hard to tell if you can simply add a resistor to it,
the more important thing would be: what kind of controller do you have, some come with a throttle limiter some are programmable etc.
And the sensor should tell the controller to stop, not cut of the motor itself;
mainly because the sensors are all low voltage low current (just as the controller inputs) the motor is not
It can be inline with the throttle via a relay;
but again, maybe the controller is the better alternative if it comes with a kill switch connector.
if you cannot find 24V bulbs (which I think is by far the best option you have)
you might get away with just a resistor set up
that is if you have LED lights and indicators...
(it'll draw some power, yes but it works and is relatively cheap)
The next best option is the 24V to 12V step down converter, but find one that's up for the task (some do not provide enough watts to run lights)
The last I'd do is get a second 12V battery.. that'll end up being more expensive than a converter with all the extra cables, mounts, chargers etc.
'sid
That's a sign of how small the bus is.....
I cant wait to get to the fun bit of putting them all together.
I think I can see where you're going with this.....