Is there an OpenWRT feature or package I can install that would allow me to track internet data usage per user? I'm thinking of a system that requires all devices to login with a username and password so their internet data usage can be tracked regardless of device.
take a look at this: wrtbwmon.
A small shell script designed to run on linux powered routers (OpenWRT, DD-WRT, Tomato, and other routers where shell access is available). It provides per user bandwidth monitoring capabilities and generates usage reports.
You might need only that, but i haven't tried it yet. I got the reference from here: Bandwidth Monitoring.