Can you rig up something that works mechanically as a fail safe, with the best will in the world something will break on your car at some point
and you can guarantee in your case that it would be something to do with your gearchange! and you would probably be as far from home as you can possibly get
If you could have it electrical and mechanical you can physically put it into neutral and then use your solenoid for up down which may make it a little easier for you