BTW I asked
Velotech Cycling Ltd. about this, who are the main UK Campagnolo Technical Centre and got this lengthy answer:
"You are correct, the combination that you want to run does exceed the designed capacity of the derailleur with a frame and gear hanger that are compliant with the Campagnolo spec.
You would need to run a chain 1 link longer than optimal to be safe in "big to big" bit this would mean inadequate tension in small-to-small, with the result that you would have a slack idler run on probably the smallest two sprockets (depends on frame geo how bad this is).
Slack chain on small to small is not in itself a disaster BUT - it can lead to quite a bit of lateral whip in the chain and an increased risk of the chain being caught on the back of the big ring and giving a chain-suck type effect which can be a bit disastrous for your paintwork (and in an extreme case can lead to a chain so thoroughly jammed that you end up bending a chainring and / or having to drop the crankset off to free the chain.
The fact that Campagnolo launched 12-27 after the max sprocket and capacity information was released is largely irrelevant - these things are not governed by production, they are governed by a combination of frame geometry, exerted chain tension, front and rear derailleur design / geometry.
At one time, Campagnolo made a compact crankset with a 48 outer in 10s, for instance, which could be used with a 12-27 cassette if a compromise was made in the setting of the "H" screw, or on bikes with a rear derailleur hanger at the upper end of the hanger length specification. That could actually be worked around by changing the H screw drive ratchet for a longer one from the 11s component range ... it all depends what compromises a user is willing to make on function and how much modification a mechanic is competent to make and in new material, whether warranty is an issue that matters to the end user. "