It would be handled like any other IFR flightplan with a VFR altitude. It would get sent to ATC. When you call Potomac on the telephone to get your squawk they'll ask you what gate you're departing. Then they'll modify it. They may catch the clue that you're looking for flight following and put something in the system. If you want flight following, don't do anything special on the flight plan (ie just enter the gate) other than entering your desired cruise altitude, but let the person on the phone know. That way they'll give you an operational frequency, rather than the frequency set up for VFR sfra operations which does not officially provide radar advisories (they use the magic words "transponder observed" rather than "radar contact"). Sometimes though they'll just tell you to let the controller know (and you'll just negotiate with the SFRA controller for a frequency change for flight following.