I would call the airline - and document it!
Write down the date, time, the employee and if available employee id number. Then ask them about a harness. If they don't know, ask to speak with a manager and ask where to find the info in writing (ask if they can email it or fax it to you!).
It may seem like a lot of hassle, but I flew with a toddler and when the flight attendent approached me about the way that my child was flying, I had documentation to back me up.
Good luck, and I would recommend taking the car seat anyway, most kids are more likely to sit still in one. Plus, it can double as a feeding chair for snacks in the hotel room.