"Does this mean that a UK/JAA ATPL who has an FAA PPL (full or piggyback) but not an FAA IR can fly an N-reg aircraft worldwide IFR?"
No! You can only fly the N Reg aircraft in the State of Licence Issue on the JAA ATPL. You can exercise any of the privileges of the ATPL in that State. A JAA ATPL is only valid in other States in JAA registered aircraft if the National Authority permit it.
Outside the State of ATPL issue you can only fly to the privileges of the FAA PPL.