Can't say I've restored WAS miniatures. However, hydrogen peroxide is a popular fix for old Star Wars toys like a vintage X-Wing Fighter. Considering the Star Wars toys I've seen restored are usually much larger and all white, not sure how this would work for miniatures that are painted.
I have a few planes with yellowed bases. The clear plastic turned yellow. I don't think I will do anything to fix this. As the prices on these minis go up, I'm not sure I want to mess with them.
Besides the video I posted above, there are a ton of articles about plastic toys turning yellow on the action figure collecting pages. I guess the solution depends on the cause. There does not seem to be a clear consensus about oxygen, cheap plastic, uv rays, smoke, etc.
I am sacrificing a damaged Devastator for knowledge. I coated the base in high concentration hydrogen peroxide and left it outside. Will update with results.