It turns out the answer is: don't use the SourceForge docs! The link I gave above gives utterly the wrong directions!
The repository I created above is cloneable from the URL hg clone ssh://<user>@project.hg.sourceforge.net/hgroot/project/newrepo
. HOWEVER, it isn't linked to the SourceForge project page in any way -- it seems to be a deprecated location and no one has removed the stale doc.
The correct way is to go into the "Admin" tab on your SF project page, then pick the "Tools" option on the left-hand menu, then pick the "Mercurial" option under "Click to Install".
Comme ça:
And just like that, it's done!