In most skirmish games I have played, being "out activated" means that any extra models are simply activated in an order of the player's choosing with the opposing player essentially "missing" his own turn.
I go, you go, I go , you go, I go, you can't go, I go again, you can't go, I go again, you can't go, I go again.
Every model needs to be activated before the end of the turn, and this is simply the most expedient way to make that happen. Off the top of my head, this is the way it is done in Malifaux, Freebooter's Fate, and Heavy Gear Blitz. Some games include a "wait" action that allows a model to prepare a conditional action, like a reaction. This would be like Overwatch in GW genre games. Other games, like Dust Warfare, Infinity, and HGB include reactions as part of the normal activation and action sequence.
BWG does not have a reaction system, but it does have a both sides attack in melee system, although that is also very common, particularly in GW style games (Necromunda, Mordheim, Gorkamorka, etc.).
In my campaign where the players will typically be outnumbered by enemy models, I have put in what is effectively an XP based activation order. The GM activates a number of models to equal the average XP of the players' models each time it is his turn.
PC activates (50 XP), GM activates 25 XP model, GM activates 25 XP model, PC activates, etc.