I just finished configuration Google two-factor authentication on my ReadyNAS for ssh authentication. It works well so far.
You need to install the Root SSH Add-on from the forums so you can ssh into your readynas.
apt-get update
apt-get install mercurial make gcc apt-utils libc6-dev gdb libtag1-dev uuid-dev libpam0g-dev
The just follow these instructions
http://www.mnxsolutions.com/security/two-factor-ssh-with-google-authenticator.html