They offered 1/2 deposits for much of 2009 and at least the end of 2008 even for bookings not made onboard. Since that time, the onboard booking has been the only way to get that benefit on a new reservation.
However, they picked a very bad time to stop when they finally stopped that offer, as they did it between the time when the Platinum and Gold CC members could book Alaska and other new itineraries, but before Silver CC members and the General Public could book. Not sure if the flax they got over it would make them more or less likely to offer it again.