I've never seen one. Only thing I've seen are extenders and 20->24 pin adapters but none I've come across are right angle. I suppose you could just get an extender and bend the hell out of it.
Why would it be? The only change would be that the wires would come out of the side instead of from the top.
I think this is a great idea actually. The only issue is that the orientation of the connector varies depending on the board. If it was standardised though, it would be a very useful item to have.