The admin theme bar works great! It is exactly what I needed with one exception, themes that are unique to a multisite blog are not displayed.
Enabled for everyone
Enabled for movie_blog
In this example the theme select menu would only enable the end-user to use Twentyten or Twentyeleve, not Carrington which is their site's theme. Obviously it would be a bad idea to let other people run a theme unique to another site.