ChrisT, just try to draw up your proposed wiring diagram and I'm sure you see why I was leaning towards the PLC idea. Although I've got three Shift Solenoids, so mines a bit more complicated. The timing issue of turning off switches as you simultaneously turn on others is also an issue. Here is a link to the diagram I finally settled on to get the mircro switches working:
Diagram for Manual shifting ATX! - TeamZX2.com
Maybe this will help?