Try cutting with a zip blade in a grinder. To get your patch to the right level I would use a combination of ½ and ¼ or 5/8 drywall to get it as close as possible. Tape and finish... then skim coat to get close to plaster finish.
I find that oscillating blades don't get more than a few inches done before the teeth disintegrate. At $10+ a blade, no thanks.
IMO, a sawzall creates way too much vibration within the wall, which in my experience, for old plaster, is a bad thing. I have never tried the hatchet, but to get through the metal, I assume it would do the same.
Generally, I just bite the bullet, set up some ZipWalls, break out the vac and the grinder with a diamond blade. If you keep the nozzle right up to the blade it is doable. It's pretty dusty in that little plastic room, but it's the way I generally go.
It will last 2 inches in a plaster wall around here. You will wear them toothless in 2 inches of hoarse hair plaster.Thats strange Blue... I have a blade in mine right now that been in there a while and still gets it done. Just the typical Rockwell flush cut for wood/metal. It's probably cut a couple hundred LF of plaster and sheetrock. It has a few teeth nicked out from hitting nails but it's still cutting pretty good.