Whoops. Guess 6:30 AM is too early for me to look at code.
You're right. The information isn't in scope. You'll have to first query the userfield table to grab it. Same hook.
The other option would be to add it via the model to the query hooks, but in order to do that, you'll have to edit the model. I'm not seeing a quick and obvious way of doing that without doing manual file edits.
If that's the case, though, it seems silly to have that hook in there...