dgrimmus is right.  The distance trigger is very useful for planting trees.  I have it on my FMX.  I looked at my CFX and didn't see any option there.  Just radar output and engaged output.

I guess there is three ways I see you can accomplish this depending on the true needs (tolearance of when it reverses... is a foot good enough?  2 feet? etc).

1. Get a used FMX and do the output based on distance.   You know the prices of these.  Level of difficulty: Easy.  No FIQ necessary. But you still need to close a relay for the PLC to read.

2.  Get a used EZ-500.  There was a guy on Ebay selling RTK unlocked for $500 less cabling and antenna (head only).  Purchase an EZBoom (there's in the classifieds for $700 with cabling).  Set up a field boundary and you can run through the relay on the boom switch box..  You'd have to run RTK not rangepoint you wanted to get the accuracy.  I can think of this as the simplest and easiest to achieve a true boundary with your on/off output you are requiring.  I know a guy that has an extra 500 on RTK (head only).

3.  If stuck on using the CFX-750 use the digital out to send the radar signal.  If the PLC programmer is handy, he can easily count the radar pulses and convert that to feet, inches, etc.  Keep in mind it is HIGHLY recommended to isolate the equipment via an optocoupler or at the very least a logic shifter.  The radar can be between 5 and 12vdc.  Most PLC's are expecting between 12v-24v for "logic high".  This couldn't necessarily be a boundary but would be based on pure distance or more appropriately how far the machine has moved (whether in a circle or straight line).   A few hints for your programmer here.  Any GPS source is going to have some "movement" constantly.  Whether it's .25" or 3".  Anyways, it's always going to be outputting a radar signal if the machine is stopped.  So your programmer would have to account for this.  Example.  If vehicle is moving less than .1 mph you can assume it's from the GPS drift.  Obviously, in this situation the higher the corrections source the less you should have to compensate for in theory. Also, a physical button would have to be likely installed to "start" the zero spot.  The other problem is without and HMI it's hard to change the "distance" required.  Also, high quality shielded cabling should be used and keep an eye on the distance to the PLC.

# 1 is the most expensive but fairly straight forward.

#2 is the least expensive (unless the CFX is free).  However, it would require an RTK corrections source to get your accuracy vs the rangepoint you desire.  This also gets you a true boundary and not simply a "distance moved"

#3 potentially uses the hardware you have (assuming there is already a CFX in the mix) but requires a small amount of hardware (logic shifter or optcoupler) and some of the programmer's time for the PLC... Then again, you need the guy for the logic input anyways so not such a bad idea? 

I dont' think anything less than a FIQ, EZBoom or the remote ouput of the FMX or TMX will give you on/off output.