Short answer: you can probably run the lights off the BU circuit.
If you're going to do a switch, I would run a relay. You can get a SPDT (Single pole-double throw) switch to do what you want. You hook up the BU circuit to one end of the switch, power to the other end and the wire to the lights come off the middle. So, when in one position, the lights would work with the BU circuit only. In the other position, it would turn on the lights when you wanted to. When running manually, you would have to remember to turn them off. Many states, California where I'm from included, rules say Auxiliary BU light can operate only when in reverse. If you're not familiar with them, Google SPDT switches. It will give you visual.