Okay, to assist with this script request, I had an idea come forth.
Blizzard's party-switching system apparently reads characters directly from the database, which makes all chars visible.
For this party switcher, I had the idea that the "Battle Party" (Four Characters in Battle) would be treated as a normal party, while the rest of the Party (which would be displayed in the Menu could be stored in another value. Also, the system would need a way not only to lock characters into the party, but to specific slots in some cases (in this case, if Rune is present in the party, he would be locked in the first slot of the party, otherwise another character would inhabit the slot).
Just some more info that could probably help.
I might work on some mockup images to display how "In Party" characters would be setup as compared to Out of Party characters.