As a temporary fix. If you wanted to drop the character back to level 1, you can use the Human/Dwarf/Elf/Lizard form at -1 for permanency under status effects to change them to a "new" character. From here, you can manually change their attributes/skills/talents/spells/etc to what they want.
Not ideal as they can't customise their look and it would momentarily stop the game (or leave them dead until time for a bio-break/they reach a suitable plot point) but it would work for what you want. As mentioned in another thread, the status effect would still properly show any armour/weapons they equip, unlike the prebuilt NPCs.
The only thing that might be a nuisance is (and correct me if I'm mistaken) when you move from area to area, players (in any state) are teleported to the new land, including any NPCs set to 'follow party'. So whenever you load a new level, you'll be transporting a corpse around with you. That may be what you want to do? If it was my party, I'd leave the body somewhere safe while we tried to find a way to reacquaint their soul with their body.
