I think it would be doable. Rules something like this --
Planetary bombardment becomes a new upgrade for a military starbase which has to be built in say 10 stages (let's call it Galactic Domination X). It takes one month (four turns) to charge, during which time all starbase weaponry becomes useless. Only the ship speed modules, Stellar Wake, Inverse Tractor Beam, and Interdiction, remain online. For that matter, let's have Galactic Domination increase the range of those modules by 1 hex per level, so it ends up performing interdiction over a huge area. Whether ship assist components get the increased range, or are effective while the bombardment guns are charging, would be a matter of balance.
Then, if you have a fully charged starbase in range, you get an additional option during invasion (still need the transport) -- radioactive blanket. It would eliminate 90% of defenders and change all tiles except the initial colony into red (requiring terraforming). You win easily because all the defenders are gone, but the planet will take decades to build back up.
Due to the immense hyperspace distortion involved in the projectors, any starbase having the Galactic Domination component at level V or higher becomes visible regardless of fog of war. Also due to the immense size of the structure, the exact level of Galactic Domination installed is visible without any espionage (to nearby ships for levels 0 to 4, and everyone from 5-10).
Unlike the other invasions, which have a cost associated, there should be a price for installing the modules, and another for activating the charging sequence. If you have to turn off the projectors to defend yourself, you lose the money and have to start charging all over again.