If I use the "must_be_in_party" command, the hero will be forced to stay in the party, sure enough. But you are still able to change his order, within the current party.
Like say, we have.
HERO 1
HERO 2
HERO 3 RESERVE
HERO 4
While it's true that Hero one is unable to go to the reserve... in the party switch screen, you can still reform the party so it is:
HERO 4
HERO 2
HERO 3 RESERVE
HERO 1
And in this case, the sprite for Hero 4 will be walking around the game maps, and not Hero 1, as it is supposed to be.
I suppose I could just remove everybody and then just add back Hero 1, to force him into the first position, but that is a bit cumbersome to add in a few dozen times.
I'll just make due with what I can, if there is no simple script fix to lock that player in the first position.