http://ec.europa.eu/economy_finance/euro/cash/legal_tender/index_en.htm

Within the euro area, only the euro has the status of legal tender. This results from the fact that the money used in a monetary system does only have the status of legal tender if it is provided for under the respective monetary law.

Theoretically, I suppose, Greece could issue some kind of scrip à la Wöhrl:

Yet, contractual parties are free to agree to use in transactions other official foreign currencies with legal tender status in the state of issuance, e.g. the Pound Sterling or the US Dollar. The same applies to privately issued money like local exchange trading systems (e.g. voucher-based payment systems in certain communities) or virtual currency schemes (e.g. Bitcoin). Although these are not official currencies and have no legal tender status, parties can agree to use them as private money and without prejudice to the official currency (euro or national currency) being the sole legal tender.

However, it would seem to me that it would be difficult to decouple the value of the scrip from the euro.

The fact is that what we're experiencing right now is a top-down disaster. -Paul Krugman