jj: You need to ask Brown whether the order will be routed only to the NYSE when you place it after-hours or pre-market. If so, in either case if it's a market order to buy or sell long, you will be part of the opening trade, and get filled at that price. There's no way the "specialist can screw you" (at least not in the way you mean). If it's a market short sale, you get filled if the opening print is an up-tick from the previous day's closing price (or at the closing price when it was an uptick from the previous different price).
If, however, Brown routes to ECNs (e.g. Island, INCA) after-hours and pre-market, and turns your market order into a limit order in order to do so, you could get a fill that you will not be happy with. It seems unlikely that they do this, but you need to ask them to make sure.