I suggest having the leaders and or officers have the ability to deploy a ship within the legion not actively guarding a planet. So if a legion member alerts a planet then the leadership can assign vessels to help defend when not on=line.

1: Would break TOS seeing as it would give another player control over your character (one of the definitions of multis)
2: Would be extremely abused by multis due to the fact this would make it possible to use one account to defend a planet with many multi accounts rather then all of the accounts
3: Disregarding above, would have to have a thing in settings to stop people form doing this to you similar to the setting for fellow legionaries repairing you.

Oh good God this would be a multi's dream come true...