Thanks Maru.
Well I don't think my current scenario is particularly unusual. I have a large hotel model with several cameras set up for each view. So for example cam_bar_option_1/2/3/4/5, cam_restaurant_option_1/2/3/4/5 etc. I like being able to control the WB/Exposure and Aperture/DOF for each camera within the same scene. I do this sort of thing all the time. Some of them need DOF, some not, some with different exposures etc. Currently there's no visual feedback in the VFB when I make changes this way, without re-rendering.
The functionality seems to be there already, but only when you do the initial "pre-render" thing when you hit render. Would it not be possible to make this happen without having to hit render?