Nice setup you got there. Have you ever thought of inverting the projector and mounting it from the ceiling? There are plenty of universal mounting options available for this, and as far as being mid-height at the screen - unless your projector is different the throw angle on most projectors is a straight line from the bottom of the lens, angling upward, unless inverted, then it's the opposite. This means you could mount the lens near the top of your screen if inverted and it would still fill the screen with little or no keystone error.
Would also prevent the inevitable bumps to the tripod that's going to throw your projection off, etc.
EDIT: Just noticed the last pic you posted, looks like you have vaulted ceilings.. oops. In that case, what I might do is build a supported shelf bracket out away from the wall, above the height you plan the projector to be, then use the same universal ceiling mount on the underside. This would help ensure the unit still has all the ventilation it could possibly need. Anyway, just my thoughts on a great looking system! I'm sure it's very enjoyable!