Agree that if the sole purpose of the JR pass is round trip Osaka Shikensen, it might be better to just refund it. There are series of sleeper bus from Tokyo to Osaka as well at competitive price. (which if you decide not to refund, there're some JR highway bus from Tokyo to Osaka that your pass is valid to travel with)

As a side topic, there's no direct sleeper train for Tokyo to Osaka, but Tokyo to Heimiji which bypass Osaka (which you will need to back track it to Osaka on Shikensen, but you need to pay the "sleeper surcharge")

Tokyo --> Heimiji
http://www.jreast-timetable.jp/1003/...35/036501.html

or Tokyo --> Nagoya then jump on shikensen
http://www.jreast-timetable.jp/1003/...90/094901.html

P.S. Just a side note: after checking JR West, on arrive Heimiji, you should use the normal train to go direct to Osaka Station for 1 hour (though if I hit Heimiji on sleeper, I will spend the first morning visiting the castle first)

Another Crazy work-around (though I prefer highway bus compare with this ...)
Tokyo --> Niigata --> Osaka
Shikansen MAXとき 349号 connected to きたぐに 502M
http://www.jreast-timetable.jp/1003/...70/074881.html
http://www.jreast-timetable.jp/1003/...35/036461.html