New bad suggestions for Greek collateral

Another twist in the saga of the Greek bailout and Greek collateral. The cash deposit idea was stupid. Now it is suggested that Greece nationalize some banks and post shares in those banks as collateral. Source: Handelsblatt

Seems like a good idea? But wait a minute. Those Greek banks hold a fair bit of Greek government debt. And the collateral is meant to guard other debt holders against Greek default on that same debt. So what happens if Greek defaults? The banks fail, and those shares that are supposed to act as collateral become worthless/lose alarge  part of their value. Fail.