You can see the engine mount above and to the left of the sprocket, so I'd guess that there's a enough room to slip the sprocket off. Maybe I'm wrong in this assumption, but to me cleverly placing the swing arm pivot at the sprocket locations negates the problem with chain slack.
Perhaps this would lend itself to belt drive (pivot at the sprocket), and with the room behind the frame at the sprocket, perhaps BMW is working on a fully enclose a belt drive system (protection it from mud, gravel etc.) for use on something like an F800GS? [/speculation]