Well, I totally forgot that having invalid character IDs in your save can make your game crash upon loading, rip that idea.
So, I remember someone mentioning some time ago that by marking a character with the "left at chapter 6" flag on saves allowed you to edit characters you haven't even gotten yet, however, this would not affect certain people because they use completely different blocks when they first join you than the one they'd use as playable, so I went and gathered the character IDs and hopefully we can put this to good use somehow (although the game seems to be crashing with these unit IDs on the save), do note that these IDs are recycled on all 3 routes, so be careful, as you could be wanting to have a custom block for PID_A_Silas and end up overwriting PID_B_Selena since they use the same ID.
[spoilered lists of IDs]