Mitch, I'm not a lefty but I've got two of these BM 330's and they don't have dual thumbstuds, nor does it look like one could be added easily, since the stud would interfere with the locking liner (you'd have to grind away a bit of the lock in order to add a lefty stud... probably not the best thing for lock strength).
Using my left hand, the existing thumbstud engages with my index finger, and in the open position, I could use that same finger to unlock the liner without too much trouble, tho you may have to adjust the tension on the pivot a bit. Definitely easier with the right hand.