Lets see if I got this correct
You have to replace the E stop button in the control circuit, correct?
You are going to lock out the 120 volts to the coil, correct?
You are not going into the main panel or breaker, correct?
If you answered correct to all of the above I think you are fine, you can't play the what if game.
If that were true you could NEVER work on something fed from a panel because of that "what if a wire comes off and hits the main buss or anouther breaker". You are not exposing yourself to the 480 volts just the "what if".
A cowboy may get thrown , but they always get up and walk forward.