Well, I have to agree on the Dima. I think the component holder is brilliant. I was thinking of all the features of any of these manual PnP machines, the rotating component tray is perhaps the most beneficial for the effort involved to a DIYer. Perhaps something like that could be made to attach to the base of a stereomicroscope, where the end of one or two compartments of the tray could be placed on the edge of the FOV. It would be fixed to just above board level, so that the board can be slid around underneath. Obviously, to be used on lower magnification to maintain focus on board and parts.
But the homebrew machines posted on this thread seem to have beelined in on what works for machines, rather than for people. It seems to me that instead of moving your hand around (and just your right hand, leaving your left hand to have no part other than playing the brakes) is rather backwards. It seems like you would want your right hand anchored in the same place and to just move the board (+- the parts holder) around under the hand. To this end, you could just have a floating rest to keep your hand above the level of a board. (I actually use something like this; a simple board with feet high enough to float above typical SMD components, and long enough to allow a largish board or small panel to be maneuvered around underneath, with a small tray for components on it, just on the edge of my FOV.) This way, my view through the scope doesn't change, my hand doesn't move, and my reach to the board doesn't vary. My left hand just moves the board to position, between components.
Your hands are highly skilled and dexterous instruments. They just need to be dialed down a little. Your left hand can easily maneuver the board to fairly good accuracy, just the way you were born. This leads into my critique of the fixed Z axis. Unlike a stepper motor, your hand is capable of a complete range of motion with a high degree of responsiveness and accuracy. Fixing the Z axis just gimps the hand and forces you to make fine XY adjustment with your ARM? Nooo... All your hand needs is a little gear reduction or vibration dampening to place chips with perfect accuracy. The basic pickup tool is simply a bit crude and inaccurate for what it needs to be. I have a 1 minute fix for this, and all it takes is a coat hanger and some tape. Will post a pic tomorrow. Getting late.