What if the all-ship-freeing thing happened as the attack went on, but only gradually? Every guard post you destroy, based on AIP and AI difficulty, a large portion of random AI ships are freed and sent on the attack or defense at random. If the AI runs out of ships, the AI provides bursts of reinforcements to all of its planets, and releases those ships, never leaving any planet undefended.
What it does allow, is it allows the defense and offense to be frantic and desperate, and all the more, the more guard posts are destroyed. The AI will wipe you off the map if it possibly can by diverting all of its forces as it can to stopping you. Might be worth experimenting with. If you went for the old fashioned double kill, you'd be getting double the maximum ship freeing brutalizing moments before the final blow, which could well be a harrowing experience.
Thing is, it really just sounds like a galaxy-wide stacking raid engine to me, the more that I think about it.