With a radiator that is conventionally plumbed, with a valve at either end at the bottom, it should be possible to lift it off its brackets, and rotate it 90 degrees onto wooden blocks of the correct height, so that it lies out into the room. All that's required is to loosen the connections slightly and tighten them when its lain over. Leakage should be zero or just a few drops.
Agreed, I've done that as well in the past.
Same goes for painting behind rads, nobody takes the rad off to paint they just go as far as they can with an angled brush or mini roller.
You can take a running jump with that idea. I always paint behind the rads, and I almost always take them off to do so. In the general case, it's usually possible to see bits that aren't accessible to paint.
It also depends on how the rad is mounted, a lot are mounted too close to get a roller down behind, but you'll definitely see some of the wall you can't paint.
And what about wallpaper ?
What if it's winter? You're leaving someone with no heating for at least a full day
Without heating
in that room, the rest of the heating will be just fine.
And if you've just leaned the rad away from the wall or "hinged" it down then you can still have it on - though it's output will be "a bit reduced" if it's laid flat.
As mentioned, I routinely take rads off for decorating etc - we don't live without any heating for weeks (or sometimes, months).
just so you can literally skim in about 6 square inches of plaster that's never going to be seen until you take the rad off the wall.
Pardon ? Take a rad that's (say) 4' wide & 3' high (a lot of mine are bigger than that, one is 4m wide). You'll get, at best, 6" in - and nothing from the bottom - it's harder than painting with a roller. So that's an area 3' wide by 2'6" high - or 1080 square inches. So you're off by a couple of orders of magnitude.
It's interesting that there are people advocating massive disruption to electrical systems, disturbing the connections by doing an EICR on every tenant change (could be twice/year for some properties). And at the same time there are those advocating never disturbing a radiator even for a really infrequent event like skimming the walls !