I don't think there is an option in-game, but you can use Windows accessibility features to make Alt (and Ctrl) sticky keys (enable the option 'lock modifier keys when pressed twice in a row' in the Windows accessibility options). In-game, that would it possible to make Alt a toggle.
While this would work, you would need to be careful because you can accidentally hit alt+esc to open the menu or alt + F4 when selecting the fourth party member.
If you play with a controller, it's a toggle. Maybe this could be added as an option for keyboard and mouse as well.
