I suppose it's possible to reverse two of the 3 wires in a small brushless motor, with tiny reed relays or Fets. But is it worth it??
Also, the timing of the hall effect rotation sensor is sometimes optimized for one direction, I believe.
A few years ago someone was using a 4 inch fan that was reversed to control the draft on a wood stove, but I bet that was a brush type motor.
How about mount two identical fans in opposite directions and turn on one or the other??