You could do that, but I'd be concerned that the seperation from the left/right sides. With the tweeters firing to the left/right as designed you are enveloped in the sound. With them firing down, that is a different story.
I still don't see why you can't mount them on the sides, either to the front or behind the listener. They are very forgiving. If your considering mounting them flush, your not going to lose a lot by mounting them as you should, other than needing to leave a few inches for the top driver to breath.
Can you provide a picture or drawing of your side walls so we can understand your situation?
