Since the OP posed the question, with regard to integrateds going Class A to AB, "Are there any examples of this being implemented well?" I would submit the example of Nelson Pass's INT-60. According to Pass Labs documentation, it remains in Class A until 30 watts, then transitions to AB. (The unit puts out 60 watts into 8 ohms.) In theory, the blue meter at the front center will move past the 12 o'clock position to tell the user that it's crossed into AB operation. In practice however, in the three years I've owned the INT-60, it has never, ever done so. This includes a few occasions when my friends and I were overserved with "listening fluid" and briefly turned the rig up to the "ridiculously loud" level. It never got close to passing 12 o'clock.
In other words, if the meter is to be believed, for all practical purposes, the unit never leaves Class A. Incidentally, Pass describes the INT-60 as Class AB, not A/AB. As you would expect, it runs hot, but not crazy hot. What this all means, I don't really know, but I do know that it's a phenomenally sweet-sounding unit which I have enjoyed enormously.
"...Implemented well?" I'd say, "yes."