Feeling utterly terrible today, hardly able to think straight. So of course it was perfectly sensible to test to see if the FT-857D's CAT port 12v pin is switched or permanently on during my lunchbreak. It was of course perfectly sensible to do this by plugging in a bare plug, without its shell, to gain access to the pins...
Or not. As I discovered. Measuring the pin with the radio powered but off, showed no voltage. Measuring it with the radio on, showed 12v - for a moment.
And that moment was when the meter probe slipped and shorted the pin to the one next to it - GND! It even made a nice crackling noise until the internal, and ironically, surface mount, fuse blew!
Well, I suppose I have gained the information I wanted, the pin does appear to be a switched 12v line, so I can power the RT-11 from it. But, unless by some fluke I have a suitable SMD or micro sized fuse in my junkboxes, perhaps not for some time.