Yes you can do that. As a matter of fact the NHRA Tech told me my mag kill wire had to be run through a relay so it shuts down when the master shut off is pushed off. I do not have the relay contact diagram in front of me but it should be fairly simple to use the pressure switch to trigger the mag ground when the pressure goes to zero (NC). Be careful that most pressure switches will go to ground at a low pressure, like 7 psi, not zero, so you must decide how low of pressure you want the mag to shut off. You may also need a bypass wire in the starter crank circuit so you can start the engine with no pump pressure buildup. I also have a low oil pressure idiot light in my cars which is useful to alert you that the mag got switched off due to loss of oi pressure, not some other problem. I also should point out that my oil pressure light flickers when I brake from my burnout so you may also need to address that too. One more hint - Ford makes a two terminal oil pressure switch (on their modular V8 engines) - that has one terminal NC and the other terminal NO. You may fine that typw of switch useful.
