Interesting - I am aware of two owners of U.S. flight schools who asked to have an FI(R) rating added to their PPL's - both were told, by the CAA, that they had to have passed all the writtens.
One attended a distance leraning and brush up course with Oxford - the other started with PPSC (just before they went bust).
The first of those two - has spent in the region of US$25,000 (twenty five thousand dollars) to gain the qualification as a JAA FI(R)...which includes the mandatory written exams (though he also did a JAA professional licence as a result).