I somehow missed this before; sorry. Yes, I have cut open pull valves before, but they were small CPS valves (which are still quite a bit bigger than XP valves). I don't have much experience with XP valves, as I've only ever opened up one XP (well, XXP) blaster (my brother's 175, so I could insert my homemade pump), and never did anything with the valve.
However, I should think that XP valves are no different than CPS valves except in size, so you could probably do the same thing to open it. What I did was simply take a hacksaw, and sawed all around the seam on the fat part of the valve. Here's the thing though; when you've sawed deep enough to see the inside of the valve, STOP. If you keep sawing you risk damaging the internals. Just do that all around the valve, and it should pop open. It's actually quite simple, despite the intimidation factor. Just remember to try to hold the saw straight while you're cutting.
Does that help? I can draw a picture if you have trouble visualizing it.