I looked around further and it appears Lawson might actually be easier (and there is one of these on the 5F of T1 at NRT, considerably easier to find than the 7-11 buried away in a train station). Rather than making an account and messing with sign-up forms in Japanese, you can simply save a PDF or JPG of the boarding pass to a USB drive and print it.

http://www.lawson.co.jp/service/coun...ca=ser_con_012

Scroll down to the section labeled: その場でメディアからプリント

The price is the same, 20 yen per page for B&W. If you want me to translate any of the instructions, just let me know.