I just came back to find the above advice and decided to give it a try.

First, I tried creating a quick 2 minute 3D video just to test it out. Studio refuses to render the video. The first 25% of the job (creating the frames) takes about 5 minutes or so.  The second 25% (copying the image to the output file) which normally takes a few moments takes over an hour after which the process halts at the 50% point and will not proceed further.

I tried a shorter video (1 minute) which renders completely at the normal expected speed. However, the output disk still does not flag to the player that it is a 3D disk and only plays in 2D. So what have I missed?

Experimentation shows that Studio (19) will repeatedly not render any AVCHD2 format video that is longer than approximately 90 seconds.