I used a Fons and porter washable marking pen on white weave fabric then FMQ'd over the markings. I used 8 different methods to remove the markings, then washed all the samples together with Tide on a handwash/wool setting. The best of the bunch were the Oxyclean GEL, peroxide and alcohol solutions and some expensive fabric restorer called "Retroclean". of the bunch, the Retroclean seemed to combine good removal properties but be fairly mild. The alcohol produced almost immediate cleaning but I would be scared to use it on many fabrics. Worst of the bunch was the marking pencil remover.
Conclusion: Retroclean is expensive but good to have on hand. Oxyclean GEL works great but may be too harsh on fabric!