UKH

Apologies if there is official advice about this that I've missed. I've just replaced my very old petzl verso with a DMM pivot and in all the videos I've seen explaining its use in guide mode they have an anchor that leaves the device hanging in space (not touching any rock) but they don't explicitly say if this is a requirement.

Obviously, it might not be as convenient, but is everything still safe if for instance you are belaying from a ground level boulder at the top of a crag? (see diagram)

I guess if you were doing this it would be best to have it "upside down" so if you need to give slack or lower you are pushing the plate up away from the rock not down into it?

Can't answer your question, but that artwork deserves a Like

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.

Just to clarify, you mean you would belay conventionally (not guide mode) just from an isolation loop not your harness?

Nice drawing! It is usually safe but not very practical. I've done it direct off the anchor and from an isolation loop. You still want to avoid the plate scratching against the rock. After experimenting I just reverted to belaying normally from my harness at the top of crags. 

Guide mode is best used on multipitch stances so you can faff about at the belay and eat your sandwiches. When sitting at the top of a crag the positioning is usually awkward and taking in slack takes a lot of effort. 

Yep belay conventionally just off the isolation loop.

I learnt it from an Andy Kirkpatrick book, haven't belayed directly from my belay loop since.

"...I'm very impressed with the attention to detail, the expression of the second captures a stuckist figurative quality, exemplifying a naïve rendering yet retaining an emotionally adept iconography. The leader smiling and holding their private area is knowing reference to Gilbert and George's earlier works. I think it would be most at home as an entry in The Turner Prize..."  Brian Sewell 

Can't say if best practice, but common, especially with 2 seconds. I try and make sure the plate is placed so nothing obvious for it to hook up on.

Top tip, the rope to the belay should be two strands if possible, this is normally so with a loop, to reduce rope stretch if the plate is loaded. 

I have tried guide mode at the top off a Pembroke cliff once, with the rope going over grass and the plate near the ground but not actually on it. It shouldn't have been dangerous, but in fact it was in the event because there was so much drag that when my second started climbing, I couldn't take in fast enough with him even going slowly. If he had fallen, he would have hit the ground on rope stretch (35m pitch).

As a result I will not use guide mode again when there looks to be some friction with the rope at the top.