So here are the results of some attempts to improve the linkages and connection points. First is the center mount which supports the weight of the mirror. The close-up pic below shows the bottom half of a ball-and-socket joint. The four holes have capture nuts below them. There is an acrylic hemisphere (bought from here) stuck in there which will receive the ball. The braces are these. I got them because my first go was using a smaller ball (same as the ones used for the moving points) and the mirror slid off. This is an important connection which needs to keep a 1 meter square, 40+ lb mirror from flying off in wind so the new one uses a bigger ball (with 1/2-13 threads), a beefier connection point, and those braces.
Just to be clear, here's what it looks like with the ball embedded (with a bolt in it) and the second piece attached with two of the four bolts in. That second piece is tiered so it comes down to where the ball is starting to taper in so that you can't pull the ball out of the socket once it's in place. But, the plastic piece itself is not fixed to the wood except by the braces -- that may be required at some point. All the weight is from above so it's fine under normal conditions but if a wind really wanted to pull the mirror up it might not hold as-is. So far though it works well.
So in the pic below, the center mount is now in the background. The pic shows one of the linkages that goes on top of the threaded rods. As you can see, this is also a ball-and-socket joint at the connection point to the mirror. Using the same plastic balls except they're smaller: these have 1/4-20 threads.
Below the ball you see a brass sleeve over the threaded rod, then a wingnut, then another nut holding the wingnut in place. The purpose of these things is to prevent rotation of the whole linkage (it catches on something connected to the socket mount). It's unsatisfactory.
Below that is about 3" of threaded rod with an eyebolt. I wanted to keep the eyebolt 1/4-20 and got them at the local hardware store, so the eyes are large, which is why I'm using the 1/2" nut & bolt to connect to it. That whole bottom part of the linkage is essentially a clevis. It needs to 1) connect to the threaded rod, and 2) allow pivoting in one axis only. Since I have a Makerbot it was just cheaper and more flexible to print one. The functional reason for this is when the mirror is at an angle, the connection point will be a few inches offset from the threaded rod. The trade off for this is I get to use straight threaded rods which are off-the-shelf as opposed to using curved threaded rods which is how some people solve this (including a link to this astronomer's site who does long exposure photography because it's so well done, but I've seen folks on youtube use curved rods for heliostats too). Longer term it would make more sense to do this whole linkage from stock metal or plastic parts.
So this makes some progress from last go: 1) Linkages are better (still not ideal), 2) Support point is now a made piece rather than carpentry (meaning it can be mass produced in plastic -- though it can be pretty easily done with carpentry also), 3) It's got both motors in place now so I can start to play with software more easily. Software-wise, Gabriel Miller has had heliostat tracking working on Arduino for many months so I'll be starting with his code.
When it's satisfactory, I'll post files and assembly instructions. But if there's anyone reading who wants any of them, just comment and I can post 'em.