On several types and brands cast iron housings, I have drilled and tapped a drain plug in the bottom of the housing. Many of the older designs of housings once had drains. Do it with the cover off so you can see where you are drilling (the lowest part of the housing, obviously) and clean up afterwards. I recommend a 1/4-in. tapered pipe plug, a flush type (allen, torx or square). Don't run the tapered pipe tap too deep. Try to end up with the plug nearly flush, maybe proud by a couple of threads and use a little teflon sealer to make sure you have no leaks. I made the mistake, once, of running the tap too deep and the tapered plug went halfway down the hole before it tightened. I was nervous about that, so I redrilled for a 1/2-inch pipe plug. Fortunately, there were no issues with that. Once you have it drilled and tapped, you can reinstall the cover, knowing you won't have to remove it again for a while.
Also, AutoMeter makes weld-in bungs for temp senders and I used them on metal covers to install an oil temp sensors in axles. You could use one with a plug instead of a sender. I don't think you could install it low enough to be able to drain all the oil, though. At least not on all covers.