I tried to get this award last summer, BOS-JMK, and you do not always have to fly through Athens. Air Berlin/Nikki has nonstop flights VIE-JMK a few days a week. Sometimes BA will fly from LGW, and Vueling from BCN. There are a few others like Air One from some Italian cities.. You can ticket Aegean awards on the UA site too, if you do end up going through ATH.

For flights in J, the best availability from BOS for me is usually LH (or LX) since AA left. Note that if can get yourself to DC there are sometimes a lot of awards open on AF to Europe.

I agree with mattp1987 though. The PHL-ATH is a good option if you can get it. Last summer US opened some last minute envoy awards too, so apparently they don't always fully sell it out.