Actually it is possible to get transmission speed out of the cornerpost connector. However,  I don't believe the cornerpost connector cable from Deere allows that.

When we first started making our own cables to go from the cornerpost connector to Raven Spray controllers or other devices, we had a jumper wire so the operator could choose either radar or transmission speed. We had two wires exit the harness, you could plug in one or the other to a mating wire that lead to the monitor. If you wanted radar speed, you used one of the wires, if you wanted transmission speed, you used the other.

We discontinued this practice as it wasn't used much and was confusing to the operator.  The calibration number for the transmission vs real radar was different which added to the confusion.

So I believe it would be possible to get transmission speed out of your cornerpost for the planter monitor but I don't think it would be that great of an idea.

  I would still suggest going with the addon stand alone GPS devices that are available for this purpose if you don't have radar or GPS.