I have read posts on another forum where people have used peroxide on the plastic and left it in the sunlight for a day. The results have been quite spectacular from the photos I have seen.

This methodology is well known in vintage computer circles.

Here's a couple of links:

http://eab.abime.net/showthread.php?t=39531
http://www.hardwarezone.com.sg/feature-how-whiten-your-yellowed-plastic-gadgets

UV lamp is not required except for speed; natural sunlight will do the job.

Unfortunately I have no idea if the chemical composition of pinball plastics are similar enough to get the same sort of results.