Yes, you can and I have seen pictures from there, they are good. The only problem is I believe you will have the flagpole in the middle of the picture. I have never taken them from there, sorry I can show you any, but I know someone here has some.
Probably the only time the flagpole is not in the way is for a few days around the Christmas parade taping (usually the first weekend in December). The flagpole is then relocated by City Hall and there is a clear shot down Main Street.
I tried that in May but didn't get there early enough to get in the middle of the platform. If you are off to the sides, you get trees in the way. Also the flagpole previously mentioned. In December, I am going to try to get near the Tomorrowland walkway.
You will certainly get a different prospective than many of the fireworks shots. The higher focal length you'll need will "flatten" things a bit. Note in Bob's shot how close the castle appears to the rest of Main St and also how large it looks, the fireworks will also appear to be larger and closer than in a photo taken closer to the castle.