I wrote the FLYER article.
Generally:
To Fly an N-reg aircraft, you need to have an FAA licence, or a licence issued in the country in which you intend to fly the aircraft. For these purposes, your JAA licence is deemed to be issued by a specific country, and is not valid across all JAA nations.
With a UK-issued JAA licence, you may therefore fly the N-reg aircraft only in UK airspace. Once outside that, you will need an FAA licence.
On the IMC point, there is some disagreement. The authority for determining who may fly an aircraft and the conditions they may fly it under rests with the country in which it is registered. For the US, this means consulting the FARs. FAR 61.3(e) tells us that in order to fly an aircraft of American Registry under IFR (regardless of the weather conditions), the pilot must hold a valid "instrument rating". Most interpret this to exclude the possibility for flying IFR with an IMC rating, since the IMC rating is not an ICAO recognised qualification, let alone an instrument rating.
However, this rule is widely ignored, and absent any mechanism to police it, those doing the ignoring get away with it.
2D