Im requesting this mod here because the community would benefit the most if it were added to the FAF lobby manager.
This mod works like the no expansion option, except the boundaries are lines through the map (ex: through the middle of the map on 1v1 or 2v2 maps, preventing players from attacking or scouting their opponents until time is up (eg 15 or 30 minutes).
This way you can expand to all the mexes and build up your forces then unleash them in a single battle.
I don't think a mod like this exists, but if it does let me know.