In reply to joe.vernon:
Not speaking as any authority on best practices, but as someone who more often than not belays a second using guide mode.
I've used it in the way set out in your diagram and don't feel it's unsafe, as the plate always seems to orientate itself such that it locks. But I don't think it would be impossible for it to cam into a crack in such away for it not to lock. However the main problem I have using it in your situation is that it's a pain to pull slack through when the plate is that low.
In that situation I would be more likely to use an isolation loop next to my tie in point (I use an alpine butterfly) and belay from that loop as if it were my belay loop. The advantage of this is you are not directly part of the system, so all the seconds weight goes directly on the anchors. Plus if you were needed to escape the system it's much simpler as you don't have to transfer the seconds weight off your harness so you can untie.
I also combine using an isolation loop with using an alpine up which is an assisted braking device which works on two ropes. That way if the second falls the device is doing most of the work holding them but I can still lower them.