>It's grayed out within Choice 1 >(Choice 2 is the multi-field item)
I'm afraid I'm not sure what you are getting at.
By "Choice 1" and "Choice 2", are you referring to the User Settings U1 and U2 Modes? If so, then these will contain whatever values - for items in the Shooting Menu (including Auto ISO), Custom Settings Menu and other options - you have previously saved.
To reiterate - Auto ISO is available in P, S, A and M Modes.