Well, that was not my finest hour! Using an expansion plug sounded like a good idea, but it was not. As has been suggested, the transmission output shaft extends the length of the tail housing, meaning it was impossible to plug the hole. Of course, I only discovered this after removing the prop shaft, while oil was pouring out of the transmission on to my driveway!
I threw a box under the car to minimize the oil spill (don't ask me why I didn't use a drain pan; like I said, this wasn't my finest hour!) and went off in search of my spare yoke. The best I could do was a spare prop shaft, so I set about destroying a perfectly good U-joint to remove the yoke. By the time I got it off, the oil was a big puddle on the driveway, just two days before the house goes on the market!
I anchored the spare yoke in place with some welding wire and spent the next hour cleaning the oil up as well as possible. The car is behind me right now, resting comfortable on the dolly. I'll have to refill the tranny when I get to my destination. I don't think I lost all the oil, but the bottom line is not to even think about my stupid idea of an expansion plug!
p.s. While cleaning up the morning, I of course found the spare yoke.