there's a (longish) workaround for that: load the RGB channel as a selection (Ctrl+Alt+2), go into Quick Mask mode and then hide the RGB channel to better see what you're doing (Q and then ~) and use Levels to select only the range that your interested in. Exit Quick Mask, invert the selection if necessary (Ctrl+Shift+i) and you're done
Thanks for your reply, Zeno! Instead of levels I`d rather use curves, because I would not know how to select let`s say only midtones with levels. With curves I just make a curve that looks like a U (upside down). But it would be so much easier with the histogram panel and 4 sliders (so that you can select the endpoint and also smooth transitions.