I have been controlling a K3 with a KX3 using a RRC-1258-link.
At that point (Jan 2013) the implementation was not complete.
I have been updating Elecraft (Wahne) on the additional commands needed to have a decent setup. Have not hear anything yet on the updates and whether teh current FW 1.41 includes teh updates.
Tilman D. Thulesius
SM0JZT
Sweden