Assuming there's sufficient top weight to keep statics legal, 4-5oz is about as much as you can hope for. Drill the fingers and thumb deep (and hope you don't end up with too much bottom weight), then add as big a weight hole as you can. Still not looking at more than 5oz.
I did hear of one guy who sanded his ball down from a 16# to a 15#, though I don't know if it was still of legal size afterwards.
SH