My Trimble Thunderbolt GPSDO feed the ext ref on my K3, and my HPSDR systems, among other do-dads. I have a small LCD display that is controlled by a PIC chip, fed by the RS232 output of the Thunderbolt. It keeps me posted on the status of the Trimble, and also happens to display the time....
There was a guy out of China selling them for a while, but I don't see them right now. It was originally published it as a DIY project on the web. A little Google search turned up this:
I see he also now sells an advanced kit, as well.
Of course, this doesn't automatically update the clock in the K3, but- with this staring me in the face all the time, I don't rely on the clock in the K3. I use the K3 to display other things...