Yes it is possible and yes we do have pilots who do.
To the best of my knowledge they are all on the 747F and are US based.
I cant comment on IF Cathay are required to release crew for duty.. but I do know that we must tell Cathay of any other flying we are doing (that we get paid for) and moreover with GW2, we were specifically asked to register any ANG or Reservist flying (not just the USA but all countries)