In a Custom Arena, I tried to load a mission for a different terrain than what was currently running.
When I hit "Load Mission" it asked:
"This requires the Craterma change on the host. Do you wish to set the terrain and reboot the arena?"
Sounds good. I click yes, but it then responds:
"Command not available for Custom Arena."
I tried different missions/terrain. It appears that will never work. Given that showing missions for other terrains is the default initial state, it is rude to offer a player an action that is never going to be able to complete.