I've never been able to reserve an SUV in advance at DCA, but they have accomodated my requests in person for an additional charge. If they figure it nicely, you come out ahead of what the advance reservation rate would have been. If they figure it nastily, it costs you more. But, you aren't licked if you are in DC. <grin>
On a trip this year, I asked for an SUV upgrade at Dulles and they added an upgrade charge of $30 per day to my full-size rate. I drove to the hotel, looked at Hertz.com, booked the cheapest web rate for a full-size at DCA and went there the next day to try my luck. They increased my rate to cover an SUV but not by adding a daily charge, just by applying the special rate for that car class (about $260 per week versus $130 for the full-size). The rate (as I hoped it would) turned out to be equal or below the normal full-size rate I had at IAD ($290 per week).
I probably could have kept the same one; I now know that most locations will just exchange contracts if you ask.
Charles