If event organizers don't ban unaffiliated professional photographers, then I think you're probably OK. Because here's the way I look at it -- and others can feel free to disagree. You have one client. There's no guarantee that this client would make a purchase from the other photographers based on your description of this event.
Now, if you amass a portfolio of clients for said event, or take photos of everyone and make sales afterwards yourself -- without being an official photog -- then we have an ethical problem.
If event organizers don't ban unaffiliated professional photographers, then I think you're probably OK. Because here's the way I look at it -- and others can feel free to disagree. You have one client. There's no guarantee that this client would make a purchase from the other photographers based on your description of this event.
Now, if you amass a portfolio of clients for said event, or take photos of everyone and make sales afterwards yourself -- without being an official photog -- then we have an ethical problem.