It'd be cool if neutrals started on ring or sphereworlds. This explains why they don't expand (they don't have to) and it might make them harder to conquor
It appears that you can only set where player X starts or a common starting position. So if you want a neutral to start on a specific position then you will need to ensure that the neutral is in that player slot.
Does the editor allow cargo to be placed on planets? It would be cool if you could pre-place weapons platforms on a neutral ringworld. Then it would be a real tough nut to crack!
You could leave slot 1 (most common human slot) as a normal planet, and give slots 2 to 20 Dyson Spheres as homeworlds. The AI's and any neutral players will get them.
I think neutral players come after full AIs, so if you give slot 16 to 20 stuff, it'll probably go to neutrals (or else its first-come first-served).