I have a place on the lake so i can take off from there or off my boat during April-Nov.
there are a couple of public places I think you can launch from though, get off 75 at exit 278, Glade Rd, turn left, at the Racetrack turn right on Lake Acworth Drive. About 1.5 miles after turning the road crosses where Allatoona and Lake Acworth have a spillway connection. There is a parking area at both ends of the bridge and lots of people park there and walk down to the water and fish etc.
I am guessing it would be no problem to fly from there.
Another option might be from one of the marinas, again, 75 to glade rd, take a right off the exit, go less than a mile until you get to a 3 way stop, go left on Tanyard Creek rd, take that until the end, turn right on Groovers Landing, follow that until it dumps you at Holiday Harbor Marina, not sure but they have a retail store and sell boats there so a big parking lot. They may restrict but could be worth a shot too.
Good luck