More progress through chemistry: The head bolt that sits under the head plug was also weeping a small amount of oil, and to "get me through the season," I applied some high-temp wicking Loctite. (Loctite 294) So far, 900+ miles later, including 2 hours in 90F+ stop-and-go traffic and several hours at 75mph, no leaks! My temporary fix may be permanent, at least until something else goes wrong where I actually do have to tear down the engine.

To clean and tighten that nut, I'd have to take off the cylinder head cover and the rubber plug, both of which I wanted to avoid. This stuff is oil-tolerant and wicks into the threads. It also is high-temp. I tried another version that was not advertised as high-temp or oil tolerant first, and the leak came back after the first hot-weather ride.  If I ever do need to remove that nut I may need to heat it first, but that's easy. I also tested this stuff to see what happens if it mixes with oil. It just dissolves and doesn't seem to deposit itself onto metal surfaces, so I'm not concerned about a little bit leaking past the threads into an oil passage.  Very cool product!

The stuff I used cost me about $20 for 50ml on Amazon. This was my second attempt. The first, using the Permatex equivalent of the loctite product you linked to, failed when the engine got too hot. This stuff won't melt until it reaches 450F, and it seems to displace oil - it's described as oil tolerant. That's why I chose it.

I searched around Google and Amazon for variants of search strings like "fix hairline engine crack" until I found a Permatex threadlocker product that fills small defects in porous castings. Then, after that worked briefly and then failed, I searched for "threadlocker high-temp oil-resistant" and eventually found that one. It took a while!