There are overseas FTO's in the USA approved by the UK CAA for teaching towards a JAA license these FTO's have JAA Instructors. There are also JAA examiners carrying out skills tests in the USA at such FTO's.
To do so requires the Instructor to be dual qualified in order to be paid to fly in an N reg aircraft.
However it is not possible to freelance as suggested. To teach for multi engine also requires an approved course with approved exams etc and the course is type specific.