I would be very surprised if the WMI shorts were able to file for 'negative' escrows. In fact, I worried at one time that there would be more filings for escrows than there were issued shares.
Think about it:
A Owns WMI in a margin account B Borrows shares (they come from A) B Sells them C Buys them (Someone has to buy from the short seller) in a margin account D Borrows shares (they come from C) D Sells them E Buys them (same as above) in a margin account ad infinitum
WMI was shorted into oblivion before even the seizure of the bank and BK was declared.