Apparently the chaddar - in this case the frozen river Zanskar - was still being travelled well into March this year. Two friends from Padum travelled out to Leh in two days on the 14th/15th March. I suspect chancing it is viable if you live in the area and have few other options to get out to Leh or Kargil. However, for those one-off visits for non-residents of Zanskar (i.e. if you are flying in from Europe or North America for winter trekking) a safer, more reliable date to travel on the river would still be Jan/Feb.