I played around with this last summer on a 2011 Magnum 290.  I tapped into the tractor ISO under the right had console panel and made a custom harness that went to my FMX.  In the end I ran the FMX for steering and Field IQ (rate control and row clutches on planter) and ran the ISOBUS on the Pro 700 for Pnuematic Down Pressure on the planter.  It worked but was a pain in the rear.  This year I'm not using the Pro 700 and going back static down pressure.

There is also a Trimble kit to tap into the tractor ISOBUS.  I had it and returned it.  It cost $800.