In the early 80's many Datsun transmisions had not only a reverse light switch, but also a switch that detected 4th gear in four speed models & yet another switch that picked up fifth gear as well in the five speed models. This was for the valve that allowed the vacuum to reach the distributor in only these two gears.

Since most people park their cars in first gear, I don't think that these two switches would be much good with a starter inhibitor device. Probably better to have a micro switch that picks up when the gear lever is in neutral & pulled across to one side. Only one switch to worry about then.