Good news, the characters are no longer naked and completely loading with random appearance/armour and such.
Just need to track down that pesky GUI custom option.
Those edit the two on the left. This means the customisation is fully working on all the characters. Basically, this means when the GUI is edited with another two panes, all 4 characters should be able to be edited and used.
After that would be playtesting. I currently made player3/4 run from Henchman scripts.
Edit: I think I found what I need to edit. Unfortunately, it is in .swf and not .fla which makes a lot trickier. I will try to contact a Larian dev to see if they are generous.