Workaround
My workaround is to just enter a post like this, and then edit it in the much more robust comment edit interface. On YouTube there's a "share" button that shows a code for embedding. I cut and paste that into the comment edit page, and it usually works.
The text looks like this: (iframe width="560" height="315" src="; frameborder="0" allowfullscreen)(/iframe) with the parentheses replaced by the usual HTML side arrows. But since I just cut and paste, I pay no attention to it.
Note that the video doesn't show up on the dashboard comment page, just in the original thread.