But it constitutes a savings for any who use the currency 'at maturity' to pay taxes. Probably the financial sector, if this comes to pass, will find a way to have some sort of convertible instrument/swap that earns interest as euros and then is convertible back to its digital form to pay taxes. If so the 'loanable funds' nuts will be ecstatic, and, given that the ECB is not really a central bank and the Euro is not really a sovereign currency, this might actually be true for this case. It might increase 'loanable funds' from Greek banks. The Greek central bank might treat it differently than the ECB with regard to regulations. Others could say better than I.  

"It is not necessary to have hope in order to persevere."

...opportunity cost for what amounts to a tax break for people with savings.

For Greeks able to save or invest, with an interest in both activities, and a need to pay taxes, the Greek CB could structure these digital euros like one year treasury notes, accept them for 'repo' payable in another euro pegged non-interest bearing, domestic only currency which could be digital, and then say: "Look! Loanable funds!" This would then create, through further repo, a la Repo 101, additional loan capacity equal to the total of such initial 1% interest bearing bitcoins divided by the haircut. If some of this new money leaked out of Greece as payments for imports, so long as those accepting them clearly understood what they were, where is the harm to Greece? It would fix a lot, leave the hawks with another dilemma and provide another opportunity for the Eurogroup to show its revealed preference between Grexit and reasonable economic behavior. Were it working well in Greece by next fall it could become a major problem for EPP in the next election cycle.

"It is not necessary to have hope in order to persevere."