How to restrict one knob to only affect a given area?

Hello all. Does anybody know how to restrict a ‘knob’ in a given area, such as an area drawn in a ‘rotopaint’?
With ‘Keylight’, for example, I want to adjust ‘Clip White’ knob to affect area A and ‘Clip Black’ to affect area B. I think one solution might be using two ‘Keylight’ nodes, adjust each knob in each node, and draw mask A\B with ‘rotopaint’ node separately. Then combine the results.
But that needs too many nodes. So, can it be done within one ‘Keylight’ node? Can it be done with expressions?
Any technique that can be achieved is welcomed:).

Thank you.

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