In Lab mode it's easy to change saturation independently of luminosity, by using curves on the a and b channels, but how can you do it in RGB mode? In RGB mode it's still easy to change luminosity independently of saturation by chosing luminosity layer blend mode. Photoshop also has a saturation layer blend mode, however, how do you get a saturation map to blend with?
Of course you can use a duplicate layer that you do your adjustement on, and using saturation blend mode you'll only get the change in saturation, not in luminosity, but unlike Lab mode, you can't grade on a curve, so to speak. You can't put a steeper curve on the saturated colors while keeping more neutral colors closer together (opposite of vibrance, basically), or can you?
Is there a way to get a saturation map from a Lab copy of the document, and use that somehow in RGB mode?