Jamming a TORX head bit in there might work also. Dunno what size, maybe a T45.You are probably too far gone for this but a couple of shots from an impact gun on short stroke or a breaker bar is probably what you needed to break it loose, also be sure to remove as much cack from the plug's well as you can so that you can get maximum insertion of what ever tool you use. Again yours is probably too for gone for this. But for others that are following this thread it may be of some use. But as I"m rereading your description you might try and jam the next size bigger hex bit into your wallered out plug hole. Even if you had to grind the next size bit to fit and hammer it in. But as suggested above hit it with some heat first.
^^What he said^^ If that doesn't work, heat up the housing around the plug and then try. If that doesn't work, take it to a shop that can weld a nut on it.
It may be late for this but...I did the same thing. Tried pounding in next size up still didnt work. Took it to Toyota. A tech there I know said you have to really heat the plug and housing]up well. Once heated, he pounded in the next size and got it free. Replaced it with a new one