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It's been a while since I used GIMP, but what you're doing with the RGB sounds like it'll work, and using the layer property of "colour". Maybe what you need to do is also force the L layer to luminance instead of normal. IIRC GIMP calls it "value" as a layer property. So do what you were already doing, but change the L layer mode to "value" instead of "normal". See how that goes.
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