Do you mean a patient who is actively seizing, or who is pre-eclamptic?
I very frequently give magnesium in the OR, usually 2 or 3 grams in a 1gm/2ml concentration. I usually draw it up into a 10cc syringe, attach the syringe to a luer port, and push a CC or two every couple of minutes. I go slower is the BP is soft, a little quicker if the BP is higher. If the IVF is flowing briskly, often the syringe will fill with IVF between pushes so it's always full, so after a handful of pushes - 10 minutes or so - I know the concentration of mag in the syringe is very low so I'll push the whole 10cc and then remove the syringe.
Alternatively, if the bag of IVF only has a couple hundred CC's left in it, I'll just squirt the mag into the bag. I'm not only of those people who adds a bunch of antibiotics, decadron, lidocaine, and what have you to my full bag of fluid because I just don't think that's a great practice, but if the bag is almost empty and I'm going to replace it in a few minutes anyway, sure.
This is the method I use to give pretty much any drug that you don't want to push quickly, but won't take long enough to give that you have to bother with a pump.