Since this may help someone, I just hooked up the starter disable function on my alarm. Both the internal relay provided in my alarm and my brain were fooled by the fact that (on my '99) the on with ignition accessory power is not on in the start position. ENG had wired a previous alarm into that circuit, and I never changed that.
This makes sense, since you want to cut stuff like the radio off while you're trying to get the engine to start. There is another wire that's hot during both start and run that I could have tapped into on the into on the ignition switch, but I just resorted to using the starter current to open an external relay.
'99 EB ex ENG KSWB news van, low rent 4x4 conversion (mostly fixed by now), home built interior.