I imagine a far easier option would be to use a different pin for the spindle.
The charge pump safety is another good trick, and I've sorted my spindle lockout safety as well. It'll disable the spindle while a switch is turned on, and - here's the clever bit - if the spindle is given an m3 or m4 command while the safety is engaged, the relay stays on and keeps the spindle locked out until m5 is issued and the spindle shut off. Then it's open to m3 and m4 again. Just stops the spindle running up the moment the safety is disengaged. it also locks out the drawbar motor and brake release as well, making sure the operator gets into the habit of using the safety.
A no-volt-release switch in line with the spindle would be a failsafe way of ensuring your fingers stay on, and very very simple. But I'd use a different pin anyway. You must have a couple spare.