Got it!
THIS is what you need.
Headphone amplifier, battery powered, complete with MS switch.
Plug into the line-outs of the FR-2 for perfect monitoring.
The FR-2's headphone amp is pretty poor and was designed for lowest battery consumption - this will by-pass the internal amp and give you a decent headphone amp. with MS monitoring.
The Pearl unit mentioned earlier is just an in-line transformer to use with the existing headphone amp.
I hope this helps.