When a player has a "full" squad selected (all units in the squad, or all of a filtered type, or a full group intersection) and a new unit joins that squad (e.g. a lab finishes building), automatically add the new unit to the current selection (except if that unit is in wait state).
This would solve the reinforcement problem: if you select your land reserve via group intersection and a new grunt comes out of the lab, it's immediately part of your selection without needing to re-press the hotkey.