Here's a shader that let's you enter a specific brightness multiplier for the highlights.

I used a Bias node again to make the mask directly from the source image, but you can use any mask you want.

Note the Color_Math:Max function to combine the boosted version with the original. Whichever is larger is used.

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Renderosity forum reply notifications are wonky. If I read a follow-up in a thread, but I don't myself reply, then notifications no longer happen AT ALL on that thread. So if I seem to be ignoring a question, that's why. (Updated September 23, 2019)