They are called "P leads" because they are connected to the magnetos primary winding. It is true that when the mags are not switched off, these wires are part of an open circuit, but they are still electrically connected to the magneto coil primary winding circuit, and therefore do carry alternating current voltage. Grounding the P lead obviously shuts off the mags by routing all of the A/C current to ground and away from the coil. I would not reccom end result naming the P lead wires through the same feed through as instrumentation wiring, but it will be just fine if it is routed through a feed through along with the main buss and alternator control wires.