Custom scenarios require that each player be given a slot directly, not to teams.
The feature you're looking for is [scenario distributions](http://wiki.warlight.net/index.php?title=Scenario_Distribution
). They can do exactly what you're presenting here since they allow defining of map regions that are owned by a single entity, whether it be a team or player. This is how Double Earth's Top/Bottom option works, for example.
Unfortunately scenario distributions are currently not settable by game creators and only map creators. So you'd only be able to do it on your own maps, or ask a map creator to add a scenario. If you'd like to see this ability given to game creators, you can vote on it [here](https://warlight.uservoice.com/forums/77051-warlight-features/suggestions/1941129-allow-game-creators-to-define-their-own-distributi?ref=title