I've been following this thread for a while and been interested in the many solutions. Mine comes from the days (25 or more years ago) when I dabbled in European HO. The Austrian firm of Roco had a close coupler (which I think they got from Rowa, via a takeover) and this enabled long corridor vehicles, with rigid gangways, to be coupled, touching, and work around 15 to 18 inch radius curves. They looked ridiculous going around such bends but it worked, perfectly, always. Now I mention this because that basic mechanism is the same as Hornby now use, and, I think, Bachmann. I only have experience of Hornby so far. I presume patents have expired, because the Hornby close coupling heads are almost identical to the Roco ones. Indeed, they're interchangeable.
I have them on the Hornby Maunsells which run with the gangways - with the end boards removed and any rough edges smoothed off - touching on straight track and gently opening out as they go around curves. As supplied the Hornby couplers still leave a slight gap, which you can cure either by replacing the heads with Roco, or by slightly shortening the end of the coupler pocket and gluing and pinning the heads in. The most important thing is that the actual coupling between vehicles must be a rigid bar or the expanding mechanism will not work properly, as it relies on the bar moving from side to side to open up the gap. Substituting a Kadee gives a flexible bar which counteracts the mechanism to a degree. Tim's pictures of the Keen mechanism shows a very similar principle, although it isn't lightly sprung as with Hornby. The great thing I've found is that with the gangways lightly touching - the light springing enables this - the whole train moves as one; the bogies go their own way and seem to stay on the rails very well.
I have yet, however, to try them on superelevated curves, but so long as there's enough weight to keep the bogies down on the track, I don't think there would be problems. I have had a test rake literally hurtling through the crossovers on 'St Merryn' with no problems. As for appearance, Tim's comments apply, and it's not too good an idea to turn the coupler heads upside down, as they can foul the buffer beams. I shall have a go at thinning them down a bit to see if this can be done, but this might make them prone to coming apart inadvertently. Alternatively, some dummy pipes either side of them should lead the eye away.