There is a recipe for 5.2 about overriding a web filter, which you can find here.
I used the scenerio in the recipe to look at the logs. The best place to find information about who was using the override was in the Forward Traffic log, where you could see one user (bwayne) being blocked, while the other user (ckent) was able to access a website that would otherwise be blocked. I've attached a screenshot of my results.
In 5.2, once a user authenticates, they automatically use the override if it has been set up for them. This means that they can use their regular account for authentication.
Technical Writer, FortiOS
Let me know if there's anything you want to see added to the FortiGate Cookbook.