Ah, yeah that's what I figured... too bad you can't attach PCO's to vehicles server-side
As far as I know you cannot get the Zippo turret to work server-side. I think the reason behind this is that the turret isn't technically a separate PCO. What I mean by that is there's only 1 entry spot on the Zippo, and that's the driver seat. The Zippo turret is somehow tied directly to that driver seat so when you don't spawn the entire vehicle it doesn't really work. I know from experience, I tried spawning just the Zippo turret in the past... and as you're already well aware it'll show up... but you cannot enter it.
Any other type of PCO that counts as different position (ex: BRDM turret = pos.2, BM21 turret = pos.2, AC47 minigun = pos.3) should work. The problem with those is that you cannot be killed out of them. The game developers never intended for anyone to be killed out of the Tango turrets without the entire ship going down! Exceptions to this rule might be something like the MUTT turret... but I haven't verified this.
Good luck, let us know what you find out.