I found this spot last night while walking my kid on a stroll from BWV and MGM. To see Epcot's fireworks, walk down the path towards MGM and stop maybe 20 yards from the tennis courts/MGM sign. Where the paths fork, never to run parallel again. Look towards Epcot of course over the BWI roof.
I was walking my kid and notice a security cart stopped on the path. Low and behold was the fireworks in the full glory. No one else was there so I guess it is an unknown spot. I even think it could possibly be better than standing on the bridge near the gates to Epcot. I do believe I saw more 'works than on the bridge.
And no, the fireworks did not wake the kid. But for the life in me, I couldn't get him to go to sleep before hand. Funny how that works....