chased down a plug (i bought two) from a bit away. when i opened the hood (it was daylight, whereas when the problem arose it was almost dark) i could see that the plug did not have a hole in it or any sign of corrosion. i could see part of the lip of the plug proud of the recess along a portion of it. so, i used my rubber hammer to seat it back in place. replaced the coolant that had spewed out, and no leak. drove it back to my old house, still fine. there it sits until i get a ride to bring it back.
i surmise that numerous heat/cold cycles, and the difference in the materials (cast iron head vs steel plug) allowed the plug to gradually work its way upward until it finally leaked. anyway, i'll keep a close eye on it for a bit to see if it stays in place, leaks, or migrates upwards again. i've got two spares if it does. when i get it home, i may give it a bit more solid whack with a blunt drift using my drilling hammer to expand the plug a bit more firmly.