Also, Martin suggested we migrate to Python's svn and then go along for the svn->hg ride. Does that still make sense now that some planning has been done?
I don't think so. It should not matter from which repository the conversion is done, especially since there is no shared history at all.
Also, I would not merge the Jython SVN into the /projects repository, but instead set up a separate repository (http://svn.python.org/jython, and svn+ssh://jython@svn.python.org/).
As discussed at PyCon, we should collect SSH keys for Jython committers in advance of the svn switchover; we would have to do that either way (also for hg), so now might be the right time to start doing it.
As for the repository organization: I have now opinion (or, rather, no experience with hg).
Regards, Martin