I think what Otralot is saying is that the OP could, theoretically, buy one roundtrip as follows. Let's assume the OP's wedding, reception, rehearsals, etc. occur betwen Oct. 1 and Oct. 8 (just so I can construct a clear example!)

NRT - MSY October 1
MSY - NRT October 15
Fare = $1200

Then, OP could buy a second roundtrip as follows:

MSY - NRT October 8 (OP can now return, as the wedding is over, right?)
NRT - MSY October 12 (which would theoretically get OP back to MSY in time to catch the original return flight back to NRT, booked for the 15th.)
Fare = $1200

Thus, Klaxon would head to MSY as booked on the first roundtrip, get married, celebrate (congrats!) and breathe a sigh fo relief. THEN, Klaxon would return to NRT on the "outbound" portion of the second roundtrip.

Remember, your excuse (if caught) can be: "I must return to NRT for quick business from Oct. 8 -12, then be in New Orleans from Oct 12 - 15, and then fly my original return legs from MSY - NRT as booked on Oct. 15." In other words, you will begin using the "outbound" legs of the second roundtrip before any legs are cancelled due to your failure to show up. You will be back in NRT before you fail to show up for the first throw-away segment.

Certainly, CO discourages this practice -- and you would perhaps run into trouble if portions of your itinerary were cancelled before your necessary segments! However, I have on SEVERAL occasions made analogous trips as follows:

IAH - CDG - IAH (where I was in Paris for, say, 4 weeks). BUT, while I was there, I had to made a VERY quick 3-day trip to IAH, even though my "other" return portion to IAH had not yet been flown. Thus, the answer is YES: If you are in your destination city, but you need to make a quick trip abroad, then return to the destination city and catch your original flight back home, you can do so.

But you may some explaining to do, so make sure your story is legit!