I think using NFC ring for using Nxt is a perfect choice. Nxt devs will help you with any questions regarding integration. Right now, useful info about Nxt can be found here: https://wiki.nxtcrypto.org/wiki/Main_Page or whitepaper: https://docs.google.com/document/d/1BWVPUsyS6G4gIyPXNO_XFoszhzP6jJRGe8t--RtpMkY/edit