If the goal for such aliens is to have a region in slace ionized, then they should skip forming the oxide. They should spread O2 in one region of space, and metallic or free magnesium in another. For the oxygen region, simply shoot elecrrons at it. For the magnesium one, heat it into a plasma.
How useful this would be would depend on the actual endgame of the aliens. If the goal is really just to have ionized space, this is an exercise in futility. Nebulae are usually made of ionized gas, but they have neutral components too, and gravity binds the whole cloud together. A mass made only of particles with equal mass would not even form a nebula - it would spread to be as thin as the interstellar medium at best. It would be practically impossible to measure any charge. As a matter of fact, the denser the initial state of each region is, the faster the ions will be shot apart from each other - they may be accelerated to fractions of the speed of light. Most of the mass might escape the galaxy in a few millenia, so civilizations like ours would not even know that such an event took place.
However, if you clump that material tight enough for its mass-energy to overcome the electromagnetic repulsion, you can have a charged black hole. This is a less extreme version of XKCD What If no. 140, which was considered by Munroe the most destructive What If.
We have a problem here. The mathematics involved are beyond rocket science, but this piece of info from Wikipedia is all we need to focus on for now:
Black holes with 2rQ > rs can not exist in nature because if the charge is greater than the mass there can be no physical event horizon (the term under the square root becomes negative). Objects with a charge greater than their mass can exist in nature, but they can not collapse down to a black hole, and if they could, they would display a naked singularity. Theories with supersymmetry usually guarantee that such "superextremal" black holes cannot exist.
And in our case, the charge is greater than the mass.
In layman terms, a naked singularity is a black hole from which light can escape. Therefore we would be able to look at its inside. The XKCD What If I linked above has this To say about naked singularities:
Once you have a naked singularity, physics starts breaking down in very big ways. Quantum mechanics and general relativity give absurd answers, and they're not even the same absurd answers. Some people have argued that the laws of physics don't allow that kind of situation to arise. As Dr. Keeler put it, "Nobody likes a naked singularity."