The seal should be accessible from the outside of the case. There are several different methods of removing a seal.
1.) If you have a slide hammer with a sheet metal screw on the end, you drill a undersized hole in the seal, insert the screw, slam the hammer and the seal should pop out.
2.) Drill a undersized hole on each side of the seal, thread screws in leaving about a 1/4" proud, slip a pry bar (lady slippers if you have them) behind the head of the screws and pry out.