Unfortunately people on unpaid leave can be recalled to work with minimal notice, and have been. The 900 hours limit isn't going to be an issue if the downturn is anything like as bad as expected as 747s will be parked to be replaced by 777s (freeing up 3 crew members) and long haul flights are reduced with an increase in short haul flying to safeguard the slots. I would imagine the very last thing on the minds of manpower planning is the risk of being short of crew next year, and thats before any crew productivity increases are sought by management.