(I am trying to imagine how issuers just go right on doing their thing on waking up one day to find MPEx has vanished...)
-MarkM-
In the unlikely case that the continuation of MPEx becomes impossible, all issuers will be provided with lists of their shareholders. A successor system would have to be constructed (either centralized or maintained independently by the respective issuers), but no information is lost : since people can still sign with their keys, it is trivial for anyone who is in fact a shareholder to identify himself as shareholder. Just to be on the safe side, a dump of all users' final STATs, encrypted with their keys and clearsigned by the exchange will be released before closing, in whatever manner will be deemed at that time efficient, as for instance on a p2p file sharing system.