You are forgetting something important. The spring and the hole that vents the bolt, that's why it's safe for your finger or tongue to be placed in front of the bolt. Remove the thumb screw, unlock your valve and air it up to see what happens. With nothing to prevent the air from bleeding out the end of the power tube you've just created a reverse firing marker that fires a piece of aluminium with 400 or so psi behind it. Now steel braided cable might be enough to stop the valve but I doubt macroline would stop the valve.
I've seen what happens when someone doesn't secure the pieces in the back of the marker when the drunk step father of a friend decided to air up a spyder that was partially disassembled. It didn't hit anyone luckily....
Using something that isn't rated for the tolerances required is just asking for trouble... It'd be almost like inserting plugs into the burst discs on your tank regs... They hold don't they? How often would it be a problem? Not often, till the right circumstances line up.. then you have more trouble than you want.