alas you are booking late for summer 14. my advice would be to find transatlantic award availability on a united partner, and then add a revenue flight into greece, or add on an Aegean award flight. on the return, you may get greater availability if you do an overnight in your transatlantic gateway. for example, an evening flight on Aegean ATH-LHR, and then LHR-EWR-BOS the next morning. United.com won't suggest this option automatically, but should price the same if you do a multdestination search.