We have a winner! Maybe. I was preparing to swap in a different crank tonight, but before I did I tried one more time to reproduce the noise off the bike. I pushed and pulled as hard as I could on the left pedal and every once in a while I could make it make a little tink. In my hand it felt like it was coming right from the pedal and not further down the crank.

Now... I absolutely applied lube to the pedal threads every time I changed pedals on this bike, but I thought it was worth trying this easy thing one more time quickly before I tried anything more difficult. Took off the left pedal and cleaned the threads on the pedal spindle and inside the crank arm thoroughly, I applied some lube, put the pedal back on and now I was not able to make it make a little tink. I took it for a spin and the noise is gone.

I hope that it was just some dirt or grit in the threads, and that the problem is gone for good. I think I can't really rule out stress cracks in the crank arm, though. In the past just getting it wet made the sound go away, too, so good lubricant in the right spot should do the same thing whether it was grit or the beginning of stress failure. Regardless, I'm happy I at least know the source of the noise at this point. If it comes back, I'll try a new crankset. (I've been thinking about going to a 48/34 anyway.)

And yes... I'm very embarrassed...

Thanks for being a sounding board.