Yes, you would use a bit of square stock as a bit to turn the plug out using your 8 point socket. The difficult part is finding the right bit of 5/16 inch square stock.
The first thought would be to stop by the auto parts store and get a piece of 5/16 key stock, but that will not work, it is too soft, and will twist like a pretazl.
You need something made of a little sterner stuff, something like a high speed steel lathe cutting tool. You can find this at about any industrial supply house. Buy a 5/16 square tool bit blank. It will be about 3 inches long, and have one end cut back at a 45 degree angle. Old time tool and die makers used these to grind their own cutting tools for lathe work. You will cut the end off of it to make it long enough to bottom out in your socket and still stick into the plug.
If you have a local machine shop you might stop by, explain the problem to them and walk out with a gifted piece of 5/16 tool stock about an inch and a half long, it would be something they were getting ready to recycle as it has reached the end of it's effective life and it now too short for them to grind a new cutting edge on and use again.
Note; you can't cut this with a hack saw, you will have to use a die grinder or dremel tool with a cut off wheel.
A propane torch won't work for two reasons, A. It will not get hot enough, and B, most of them fall into the blow torch varity, and do not make a fine enough flame point to direct heat where you need it.
A MAPP gas torch, available at any True Value, like a propane torch but 1000 degrees hotter, might do the deed, the issue is going to be getting a small (size) flame point so you can just heat the plug, and not the engine, and your forearm.
Next question, Are you sure that what you are thinking is a square 5/16 socket, is not actually a 7/32 female hex, like you would use an allen wrench on? I only ask because I have gotten caught in this trap before only to find out after burning all the crud out of the hole that what I thought was square thing was a hex.