Neither the MR-71 or the FM-3 have external de-emphasis switchability (as you already know), and the correction can not be accomplished via interconnect. The change to the circuit must be done internally, and with the FM-3 at least, a series of capacitors and resistors must be changed, as shown on the circuit boards above, indicated by the red arrows.
The circuit schematic to change the FM-3 from 75 µS to 50 µS may be available somewhere, but you'd have to know your way around a soldering iron, even if you do find it. I can't speak for the MR-71, which may involve changing a single resistor, but I have my doubts about that.
I think the easiest route would be to do as itman suggested and find yourself an FM tuner with an external switch, such as his Pioneer F90 has.
The good news may be that FM tuners are often available now at nearly-give away prices. Visit the Tuner Information Center (link below) and they may have tuners with externally switched de-emphasis listed, to make your search easier.
