Taking Gene's good advice I set up the K-x with trigger as shown in the next photo, determining focus manually as best I could. I used the FA43 again, but this time at f/11. I should note that the K-x viewfinder is noticeably less useful for MF than the K20D I am so used to. It's harder to see detail and it seems that the hexagon indicator has greater latitude. I could get it to light with an image that was way out of focus.
K-x with flash trigger (Shot with the FA 77mm Limited.)
Next I set up my flash on a little tripod, using a convenient coffee table. I set it to Manual mode and made sure it was full power, then bounced the flash almost straight upwards for a room fill.
Metz-58 flash with trigger (Shot with the FA 77mm Limited.)
I set the K20D on ISO100 and magically the exposure was perfect the first time. My procedure was then to increase sensitivity by one stop as I manually decreased the flash power by the same. ISO200 paired with 1/2 power, ISO400 with 1/4 power and so on. Exposures looked pretty darned consistent for this series.
Then I did the same for the K-x. This time, for some unknown reason, I needed to change the positioning of the flash to get ISO 100 exposed properly. I estimate it was 1 EV under. I then took a similar sequence of photos.
I'll post the results when I have them.