In the interest of having the possibility for escort missions where the critter being escorted has no attack powers, and thus won't (theoretically) aggro on hostile critters and go running off to be defeated...
I suggest the following alternative:
If a custom critter is created with no attack powerset they are flagged to be worth 0 XP, 0 Inf, and 0 Tickets on defeat. To keep the "buffbot" problem down, one could also have any rewards earned on a mission that includes a no-attack custom critter divided by the number of those critters in the mission design. Lessening or negating Recipe/Salvage/Vanguard Merit drops for such missions would also be a good idea.
After all, it's only one mission with "nerfed" rewards. The loss wouldn't be greatly missed, I would think.
The 0 Xp/Inf/Ticket thing could be extended to the Cimeroran "Immunes Surgeon" or Council "Transformation Capsule" Bosses and similar, instead of making them totally unavailable.
Admittedly it is probably easier/faster to just make things unavailable (And prevent exploitation) then try to implement the above suggestion at a later time, when circumstances allow. (Re: Priority List)
So this could be a "pointless" suggestion, in which case I look forward to having the freedom to include these concepts in custom Missions.
Cheers.