I don't know if this is cheating, but I found a bug yesterday to keep Shoon in the party without being able to take him out. First reach the part in stormfest where you chase the armes soldiers to through the forest. Then instead of fighting them head north to the shack and the back road trough the trees to the entrance of the forest. Then head to the western palace and go to where they would have kept the Twilight rune. All the sudden the game thinks your past all the stormfest story and you in the east palace.
Shoon will stay in your party without being able to take he out. I got up to the first war in Rainwall, but I did not bother to continue past that.