Try give the plug a gentle head on tap, emphasis on gentle. Thats always worked perfectly for me on bolts and screws in aluminium castings. If you cant get to it put your socket/allen key in.As i understand it think the two materials react and kind of bond together a little so a gentle shock brakes that normaly you see that white aluminium fluff in the threads.
Ive had bolts that feel like they will snap ,the tools will snap, the casing will snap, giving them a gentle tap and they came off with little to no effort.
Try an impact driver, you wack it with a hammer (or rubber mallet) and it turns as the impact arrives, used it on many motorbikes to undo screws and bolts in aluminium casings following corrosion of steel/ally which can be a bugger!
