You can register at btc-e.com, it will provide you with a namecoin wallet address.
It is not designed specially for that, but it works great.
I wouldn't recommend using the wallet address of a exchange. For that matter even storing any coins in the exchange.
Usually, exchanges are the first place that the attackers target and the hot wallet gets hacked more frequently. Another risk is that what if the exchange is shutdown by law enforcement authorities.?
Paper wallet is always better. However, if you need an online wallet. Go with a site which offers only the wallet service.