I got trimming to work on my laptop with Compiz:
Need to see if Xgl won't ruin it though..
.. and after some testing, it doesn't. There are some other caveats though. The regions I am blowing away still represent logical pixels, so if I remove them from the middle of the screen, I leave a giant hole.
On the other hand, I figured out how to draw DUPLICATE screen areas. So we could maybe draw duplicated images in the overlapped areas, which would look coherent (assuming rotation is good..) albeit being brighter as well.
And actually, this is WITHOUT a plugin. I found it after talking to another Compiz developer.
11 years ago