Well, I'm not sure what to make of this . . .
Between the time I took my car into the dealer, and when they actually got around to working on it, the engine gremlins apparently fled, because yesterday the engine ran fine, and when they finally got around to putting it on the
PIWIS, there were no fault codes stored (despite having had a flashing CEL) or obvious problem. Fortunately, the tech did hear the problem when the car was brought in, so they didn't think I was just crazy.
They contacted the tech advisor at PCNA who apparently told them there was no point in tearing down the engine if the problem was gone or not reproducible, and suggested that, because my car was near 20K miles, perhaps my oil was dirty and needed to be changed. I reminded my service advisor that I had already changed my oil and filter (to spec) 3K miles ago; but, I told them to go ahead and change it again so that if the problem recurred, they couldn't blame it on that. They did strain my oil when it came out and didn't find any metal or particles.
They had a service tech test drive it last night and today, and say that it is working fine, so unless the problem recurs, there is nothing else for them to do at this point . . .
