Jim,
I grew up fishing the Occoquan Reservoir. I'm pretty certain there are no Snakeheads in there and they haven't jumped the dam from the River to the Reservoir yet. This is a good thing. Some people including Google Maps will call the Reservoir, the "Occoquan River," but they are mistaken if it is any of the water above the dam.
It's a great largemouth fishery, but you may be a bit challenged attacking it with a fly rod due to the spin fishing pressure it receives. Fountainhead is too far up for smallmouth so you will be pursuing largemouth, bream, and the occasional crappie almost exclusively if you fish there. I've encountered hundreds of fishermen on the reservoir, but have only seen a handful of people approach it with a fly rod. Not that it can't be done, but definitely use a two hander or something that you can cast pretty far and work the coves and weed beds. You can rent boats with trolling motors at Lake Ridge Marina in Woodbridge or at Fountainhead Park in Fairfax Station.
If you are looking for Snakeheads and want to encounter an environment that might be a little more suitable for the fly rod, I would recommend fishing below the dam in the Occoquan/Potomac River. Occoquan Regional Park and Pohick Regional Park both rent kayaks, canoes, and maybe Jon Boats. I know the bottom of the dam in the Occoquan River and vegetation in the back of Pohick Bay can hold all kinds of species of fish and some pretty consistent Snakeheads. I've heard of people kayaking back into Pohick and hopping out to wade. If you do this, beware of snakes! Not to deter you from fishing the reservoir, but I would recommend the river (Pohick or Occ. Regional Park) for what you're trying to do.
Good luck!
Brian