I just wanted to share that I have this same problem but I figured out a workaround that might work for you as well. I was able to get around it by grouping and duplicating everything I had on the one side, then mirroring it and changing the projection settings all the way to the right. It actually projects the mirrored layer through the car and to the other side so that it displays normally. Then I just placed a big colored square over the mirrored layers on the one side so that you only see the "reflection" on the side that you can't edit. I know that's probably not very clear but it's the best way I can explain it. Hopefully this gets fixed soon, along with the other livery editor bugs.
