I have been working with Plone now for a couple of days. I think your suggestion is an excellent one. It has very crisp/clean lines ... excellent implementation...
I propose to base the Zope3 ZMI on the Plone CMF skin (designed primarily by the talented Alexander Limi and Vidar Andersen with important coding by Alan Runyan), which can be seen at http://plone.org.
I don't know if Limi & group are available to do the work themselves, unless ZC hires them for the job, but I don't think they would object either. I know at least Alan has a lot of interest in Z3.
As I'm skinning Plone/CMF for my primary job, I'd be willing to do (or coordinate if other people want to help) the work of adapting the skin to Zope3 needs.
[]s, |alo +----