That is a good point. To make full use of the bladder it would be necessary to remove all obstructions and maybe even line it up directly behind the barrel. The valve is critical. The benefit of course would be a great deal of thrust.
Now that I think about it, the pump mechanisim would have to be replaced to force more air into the bladder per pump. Otherwise you would be standing there all day pumping away between shots.
This is starting to look like a lot of work. I'm not afraid of work if there is some payoff at the end.
There is just no point in attempting to use a Magstrike this way.
By the time you've accomplished the goals you have already recognized, the only Magstrike part left is the bladder. You can't even use the clamp, because the diameter of the hose openings will not allow a quick dump. To prove this to yourself, pump up your candidate MS fully and then take your scissors to the output hose. My bet would be that it will actually take a whole second to dump, maybe even more.