I'd try putting it is as a multi city route in the search on the BA Web to see if you can even get it on a single ticket, pretty sure it would work but of course that's only half the issue. I cant see them seeing that as a valid route for a reward ticket.
You'd likely have to buy 1 leg of the trip and use the reward for the other half in the end, of course whether this is a sensible route to get from your A to B is certainly a valid debate but ultimately your choice.