Hi Ben,
Well, I'm not an EE, but I have been working on our glass toys as an A&P since '76. It won't hurt anything to ground all batteries to the yoke. Glasflugal did just that to help prevent noise in the radios used back in the day.. Make sure the ground wires are big enough. I once traced the ground in a 2 place glider to a single 20 gage wire. Also, I don't like crimp-on terminals, solder is better, but secure all leads close to the terminal because floppy wires will eventually break at the end of the solder joint. This happened to me in out ASH-25 because the factory battery plug & fuse was loose and failed after 2 years flying. We were left with nothing, not even a vario. If you don't think you rely on all our modern instruments, try turning everything off and fly 50 miles in silence. The thing I missed the most was my vario audio. Good reason to have 2 batteries, one big enough to last 4 hours with a smaller backup wired through their own master switch.
Hope this helps,
JJ