JHR - you are right, that letter from the FAA isn't overly helpful. On one hand it says that known icing conditions only exist when you see ice accumulating on the airframe but on the other they say a pilot who flys into an area where icing would probably be encountered (based on met forecasts and reports) is OK if they don't encounter ice but might be prosecuted if they do!
If potential icing conditions exist ie less than 1000m vis with OAT below zero - then as long as you have what the FAA call 'an icing exit strategy' you are pretty much acting within the letter and the spirit of the RFM.
The problem with helicopters is that once you start to accrete ice you often don't have the power/performance to climb and only a limited ceiling anyway so you are forced into descent into clear/non icing air or a turn back to a known clear/non icing environment.
So if your enroute met looks icy all the way to the deck it might be time to say no.