Graehamf, if you are still looking for a WRT build for RT-N66U, check out the 'merlin' asus-wrt builds at http://asuswrt.lostrealm.ca/
I have it running on my RT-N66U. I downloaded the wrtbwmon script to /jffs and ran that script manually with the different commands from https://code.google.com/p/wrtbwmon/wiki/Deploying and it throws some errors but it still seems to track and report usage. Looks like it uses iptables under the covers to collect statistics.
Update
Since Google Code has since gone away, I forked a copy of wrtbwmon to github and made some changes for it to work better on my Asus WRT router. One limitation is that if hardware acceleration is enabled on the router, bandwidth statistics might not be all inclusive.
https://github.com/dgonyier/wrtbwmon
The script creates a HTML page with a table that is visible at yourrouterip/user/usage.htm asfterafter it has had some time to gather data.