This actually came up for me last night. My opponent was using a Lothar Rebel with Zeb, but no turret or torpedoes. 4-LOM crashed into the rear arc, and I argued that Zeb would still allow me to shoot because 4-LOM was still within a firing arc. The VCX went boom pretty quickly after that.
Zeb as crew? Was a shuttle docked? The the answer to both these questions is yes, than a primary attack can be made out of the rear arc of the ghost, even if it is touching the targeted ship.
If it does not have a docked shuttle, but does have Zeb as crew, the Ghost can fire a secondary weapon out of the back arc.
The ghost can only fire a torpedo out the back firing arc.
If you are shooting with a torpedo or have the phantom and ghost titles with a docked phantom, then the back firing arc is active. Otherwise it is not a firing arc since you do not have a weapon to shoot out of it.
Those of you who are misreading the sheet that came with the ghost, the torpedo symbol before the word secondary weapon means a torpedo secondary weapon.
It is always a "printed" firing arc, which is what things like Autothrusters or Zeb (Crew)'s ability are looking for.
It doesn't exist unless those other conditions are met.