When I started this project, I thought it would take me a day or two. Buy some salvaged registers, strip them, repaint them, and swap out the cheap ones. As usual, it didn't turn out to be nearly that simple.
The big problem was the position of the mounting screws. Wall registers for old forced air systems were inserts in larger metal housings, and the mounting screws were designed to fit this housing, versus being screwed directly into the wall.
Once I had cut 4, 10X8 inch holes in the walls, I realized that I really didn't have a feasible way of installing the registers. This meant about 5 trips to Ace Hardware to try a variety of creative solutions.
I actually started going to multiple Ace stores, because I figured I was going to the same one too often and it was becoming embarrassing. After a few failures, the folks at Diamond Lake Ace helped me come up with a plan. The key was to use a J-hook to grab against the lath, plus a coupling to screw into. I used gel epoxy to secure the J-hook and coupling to the top and bottom of each wall opening. You can see how the J-hook wraps around the opening and grabs the lath in the picture below. In the end, I am really pleased with how these turned out. They look infinitely better than the ones the contractors used when the installed the AC. They just look right with the house and they actually work.