Personally, I think the peg on Metal Fencer's back is just to hold his stinger down while closed, in case the stinger joint ever becomes loose. It will keep the stinger from breaking off inside the tail while the Bakugan is closed. I guess we'll just have to wait and see it's true purpose.
I am CERTAIN your theory is not correct as the peg in no way shape or form is near to touching or having anything/affect to do with the stinger... it is a "peg" used to connect.