Can light GA aircraft with FDs or couplers use the RVR reduction clause on an ILS?
FAA Order 8400.13C outlines these procedures. Section 7 c outlines the requirements for this procedure. Most of these affect the airport facilities or procedures themselves, and section 5 b outlines the IAP requirements for aircraft:


  Aircraft equipped with an operable FD, or AP with an approach coupler, or HUD which is certified for operation to a minimum of 200 feet HAT are eligible for this operation.


There is also a note under 9 b:


  Single pilot operators are prohibited from using the FD to reduced CAT I landing minimums without accompanying use of an AP with an approach coupler or HUD.


So it would appear that having a FD is not entirely sufficient; a pilot with a FD can use the reduced minimums if they also have one of the following:


AP with approach coupler
HUD
Copilot