So, for the Ambient Strength trick you can turn a material to light and then set it on its on light group so that you can dial the gain as you need in Lux. One thing needs to be asked though: why? Why do you need to turn that material to light? Because if it because it does emit light, like in the case of a TV screen, then it's all OK. On the other hand if you need to "brightned it up" then that is not the way it should be done. It should be done with lighting. Often via a large mesh light that creates a bit of ambient light. 

Regarding the BG replacement look at how it's done in the movies. BG replacement is done all the time via Greenscreen (chromakey). That provides bounced light, althougn it can cause green spill. With 3D you have more options. One is to use the trick that I showed in the following video:

http://www.youtube.com/watch?v=K9ClHG0HXgg

Instead of using the sky you can simply render your scene with a backplane first to get the indirect light. The hide the backplane and re-render a second image just to get the alpha channel calculated. Then use the alpha channel to remove the BG in the first image. The technique is the same as showed in the video.

Hope this helps.

Paolo

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