Hopefully, I fixed it. I noticed that the edges of some of the access holes in the trigger casting weren't perfect. The flaws were very subtle but, with a hand lens, I could see some small areas around the perimeter of the holes with flashing that was left behind after the countersink de-burring operations on the interior surfaces. The actuator slides past these holes, though it is narrower than the internal dimension of the housing by a fair margin (it's only full width near the pivot pin).
Regardless, I decided to clean-up and polish the interior walls of the housing. I used a file, cratex bit, and a patch on a popsicle stick with Flitz polish to clean up the inside of the housing--especially around the holes. After a thorough cleaning with TCE spray cleaner, I re-lubed with KG5 and put it back together. Time will tell but I dry fired it a number of times without the trigger lockup.
Thanks for the help, hoolligan1. Your advice got me thinking that triggers can be very sensitive to contamination, lubricant, and, similarly, small imperfections in the finish--even apart from the sear surfaces, which I didn't touch by the way. The trigger already has an excellent quality so I decided to not "fix" that!