There is already a SIMILAR feature.
Esc->Manage Players: open a slot and give it a name. The newly formed force behaves as a disabled player's force. You have to directly control it, but it is completely separate from your forces.
At least once, I have played with one slot disabled as the rest of us continue the game. I give the disabled player my zgen or any other high-cost item that they can build. I carry on with normal fleet workings, then come back and take it back.
