Stay out of the shack for a few days and up pops another new version of WSJT-X.
Version 0.8 revision 3112 sports a number of improvements including CAT control (see the frequency box.)
At first I thought CAT control wasn't working because if I changed my K3 frequency with the VFO or the band up/down buttons the changes were not reflected by the software. Turns out that's how it is supposed to work. You can set the rig frequency by means of a drop-down list above the level control but it doesn't work in the other direction.
Just spotted there is a newer revision already! Download it from the usual location.