The plugs will probably rust - if you do this, I'd consider a plastic plug.
Lowes sells Hitachi diamond core bits that, at least in the smaller sizes, aren't too bad and should last at least long eough to drill your six holes - less than $20. If you buy one on-line, you can get lesser quality ones (and better, if you want to pay for it!) cheaper, even with shipping. Make sure the hole is big enough so the screw fits through easily. If you have wood backing behind, you can use a longer screw and go into the wood. If you want to use the plastic inserts, you'll need a larger hole, and the bits get more expensive. Keep in mind, the screw really isn't holding much in tension, only shear, so it really doesn't have to have super support.
The key to using any core bit is start the hole by tipping the drill at a low angle to use the edge to make a divot while running the drill slowly, than slowly rotate the drill until you've got it perpendicular to the wall. Use a spray bottle with water to keep things wet and don't press hard...let the diamonds grind the hole.