J C Whitney used to sell a cylinder head temp gauge for Volkswagens. The sender was an item like a spark plug gasket, which was used to replace the gasket on one of the plugs. Seems to me it might mount in place of, or in addition to, the gasket on the oil drain plug and provide a temperature reading. I believe a similar item is still available from aircraft sources. If you got two, you could also have cylinder head temperature. If I remember correctly it was a thermocouple type device and did not require external power, unless it had a lighted dial. True, it would not be immersed in the oil, but the aluminum engine case is a pretty good heat conductor.

Out of curiosity I did a Google search on "cht gauge". One of the sites I indirectly found is www.dakotadigital.com. They have some interesting, although pricey, items. Be it noted I have no financial interest in them.