Impossible for that scenario to happen. The camera will always receive +12v (or there abouts... ie. +13.8v) regardless of whether this is coming from the reverse lights alone, the bypass switch, or both at the same time.
Only issue that can happen with the above circuit with respect to running +12v to the camera when you aren't in reverse is the potential to illuminate the reverse lights as well. If you don't have a diode between the camera power and the reverse lights, then power can then be applied to the reverse lights when you activate the camera (when not in reverse) making the person behind you think you are in reverse.
Edit: LOL. My bad. I made that reply without reading all the other posts in the thread. I see that the problem I have stated here is the reason for the tread. In that case, give me a minute to draw something up for you.
Edit 2: From what you describe, and from knowing how people wire things up, I would be guessing that you have wired yours up something like the following:
If that is the case (or you have wired it up differently), you should try to wire it up like the following. This will work just like planned where putting the car into reverse will activate the reverse camera and the reverse lights, and flicking the override switch will activate the reverse camera but not the reverse lights. They type of diode you use does not have to be rated for much. Just you typical 1N4004 1A diodes ($0.50 for 4) will do the trick.
One thing to note is that if you want your head unit to also switch to reverse camera mode automatically when the override switch is activated (instead of selecting reverse camera manually through the head unit), then the trigger wire for the reverse camera has to be connected to the right side (as per the diagram) of that diode.
Background information for those that are wondering what the diode does when put there. The diode allows current to flow in one direction only. This is best indicated by conventional current flow (+ve to -ve) in which the arrow of the diode symbol points. Without the diode, when you flick the switch to activate your reverse camera, power also flows through to the reverse light which in turn causes it to illuminate, and also flow back to the reverse light indicator on the dash, causing that to illuminate as well.
By putting the diode in where I have indicated, you are preventing this power from your override switch from flowing back to the reverse light (because in that case the diode is pointing in the opposite direction to conventional current flow) and therefore the reverse light will not illuminate.
Thanx a lot.
This is what exactly I am after. Will try this ASAP.