Code the AI so that as human players get on the scene, they basically retreat or disappear, unless the number of defenders is less than the number of attackers.
For example, let's say 3 GV's spawn in to attack an undefended GV base. The base flashes, no human defenders respond, so AI generates several random AI defenders in always random positions to hold the line until human defenders up to take up the defense.
Once one human ups, the AI decreases it's presence. Another human ups, the AI lessens itself further. The goal being a 1:1 human defenders vs attackers ratio.
Sounds possibly workable on paper........implementing/coding it might present a heckuva challenger for HT. Might even tailor the AI response to match the attackers' GV's in capability, i.e for an M3 a 20mm or 37mm weapon, for heavier armored GV's the AI would select 88mm or 17pdr AT weaponry. The goal being just enough defensive capability to deter or slow down the attack without overwhelming the attackers with vastly superior firepower.
An intriguing idea Eagler brought up, one that I myself had thought of, glad I'm not the only player thinking along those lines.