Let me amend that: you can achieve semi-transparent DisplayRegions, of a sort, if your scene is simple enough.
That is, you can simply avoid doing the color clear (you can still clear the depth buffer), and then draw all of your objects in a semi-transparent mode, for instance with something like “render.setAlphaScale(0.5); render.setTransparency(TransparencyAttrib.MAlpha)”. Of course you would use whatever node is the root of your DisplayRegion instead of render.
If you want to set a background color without doing a color clear, just draw a big polygon that fills the entire region.
Of course, with this trick, you are drawing a scene full of semitransparent objects, which is not exactly the same as a semitransparent scene full of opaque objects. But if your objects aren’t self-occluding, it may be close enough. You can also play games with the render order to draw things front-to-back so that you don’t see objects through the objects in front of them.
David