I think, to go with this, there should be a maximum number of alert planets in the galaxy at once. It should be a pretty high number to make it a risky proposition, but it would allow for some interesting diversionary tactics - send out a fleet of raid starships to just run through a giant pile of planets, and when you hit the cap you can send a much more sizable force in to take out your target. You'll have to live with half of the galaxy being on alert for 30 minutes though.
That is what I do using fronts. On a normal map (not something like X/concentric) I tend to trow up a certain number of fronts so it takes the AI near an hour to fully reinforce them, else you can end up with every border planet fully reinforcing in several minutes.
It effectivly does the same, however there is no going back once the damage is done (hidding a cloaked fleet on a planet doesnt work ½ as well).
