I've had the same problem as you in the past.
I ended up drilling half way down the side of the seal and spraying penetrating oil into the holes. Eventually it came apart. I wasn't using the fork leg so it didn't matter about drilling down.
Corrosion between the seal and fork leg had kind of welded it in place!
Anyway, I've probably got a fork leg/slider kicking about you could have too, somewhere! Possibly not the one I drilled down either!
If you have the time, keep spraying penetrating oil on top of the seal over a couple of days. Then try again to dismantle it.