On some rare occasions this did happen, but I don't think it was very common. Probably only whilst the train was moving very slowly, or there was some special reason for the guard to be there.
I think the official rules, plus sensible self-preservation dictated that guards (and anyone else who happened to be travelling in the van) stayed seated while the train was moving. If the driver and/or guard didn't do the right thing regarding keeping the couplings tight on unfitted goods trains, there could be a very severe "snatch" at the back of the train which could throw the guard off his feet and cause serious injury (or worse if he were thrown off his train).
In the 1970s I spent many a pleasant summer evening in the vicinity of Bickershaw Colliery watching the NCB steam locos dragging wagons up from the colliery to the BR exchange sidings. Very occasionally, one of BR's unfitted coal trains would trundle off towards Springs Branch Junction with the guard standing on the rear verandah. I suspect he was there to enjoy a bit of warm evening sunshine (it certainly wouldn't be to take in the scenic views) - but this was the exception rather than the rule.
Before the use of pairs of Class 20s, the Bickershaw Colliery - Fiddlers Ferry MGR trains were usually hauled by a single Class 47. A propelling movement was used in Warrington between Walton Old Jn and Arpley Jn. Since these trains were made up of HAA wagons and were air-braked throughout, technically they didn't need a brake van on the rear for braking purposes. However specially equipped vans were provided and positioned to be on the "rear" of the train for the ¾-mile propelling movements in both directions, loaded and empty. I assume the train's guard supervised this from the van's verandah.
Couple of pictures of these MGR trains and the brake vans I mentioned are at:-
http://www.wiganworld.co.uk/album/photo.php?opt=5&id=5875
http://wiganworld.co.uk/album/photo.php?opt=5&id=10757