I don't know of any pain medications that don't affect the liver since most medications are metabolized in the liver.  Your pharmacist should be able to tell you which medications might do the least damage.

There are some alternative treatments you might try, if you haven't already.  Here is a link to an article that describes several nutritional supplements that have been helpful for people with RLS:  RLS Alternative Treatments  (Scroll down the page until you get to the Alternative Treatments section.)  Another supplement not listed in the above article that has helped some with RLS is magnesium.

Acupuncture is another option that can be very effective for pain and would be worth considering.

I hope you are able to find something that will ease your pain without further damaging your liver.