Imagine asking someone in Home Depot for help in lighting only to be told "I only work in paint." (without any other action/words).....I can imagine my response....
ive had pretty much that situation at Lowe's. and in a grocery store (probably several times. And Target. And multiple other places. Sometimes they'll page someone to come and help, sometimes the page won't yield a person , sometimes they won't and you're on your own to find someone else.
That said, I've walked up when a UA person is at a podium to be told they're not working a flight, though every time I can remember it's been pretty well before a flight and there's no one waiting so it's not a big deal. I'd probably assume the same in a situation as described by the OP. Perhaps there is a better solution - using a rope to block off the area in front of the desk when an agent isn't working the flight, or the grocery store 'light' system - where there is a light above the desk to indicate that line is 'open'.