I'm familiar with using a gray-card as a reference in post-processing to set a new white balance from my RAW images and applying them across the board. What I would like to do is identify the difference between a gray card and another surface (say an off-white piece of clothing, the palm of my hand, or colored flooring), and use that as a reference point for setting white balance in photos.

I'm not sure I'm explaining myself, so by way of example... Say I'm in a situation where light is changing constantly and there is an off-white (non-gray) wall. If I knew the wall's offset from gray, in theory I could set the white balance using the wall as a reference instead of a grey surface. To do this, I'd need to be able to tell my software what the "target" color of that surface is. In other words, instead of using a white balance tool to say "make this point gray", I'd like to use a similar tool to say "make this off-white".

I don't have a particular software restriction, I'm more interesting in knowing if this technique exists or is supported anywhere?

Similarly, it would be nice to be able to do this for exposure. Eg adjust exposure so that a nominated point is exposed to a certain level of brightness.