You can buy them. If you check in at an airport kiosk, you will be offered the opportunity to pay money to double or triple your mileage credit for the segment. On Boston-Houston, the charge is $57 to double the miles (~1600 mi).
Also, regarding speed through security... the separate status lines aren't always faster. I've seen it at IAH terminal C where the priority line is sometimes slower than the general line, though it's usually a difference of five minutes or less and only when there isn't much of a line on either side. I think it can be similar at Logan terminal A, where the general access line is a good bit longer but it also moves faster because there are more agents on ID check.
