Freezing all positions could be very dangerous as well (what if the market keeps going down or up after you freeze the positions, then a small number of unliquidatable positions turn into a huge number).  Perhaps it would be interesting to do this if market bids or asks are very shallow compared to open margin positions and/or if the exchange rate deviates from other major exchanges in a big way.  The conditions for such a freeze would of course need to be articulated very clearly and a long time in advance of implementation.