After seeing kwc's post on the his attempted redesign with Movable Type and Photoshop, I decided to go straight into the heart of the matter, by editing the structure.css file located within the mt-static directory.
After half an hour, a small number of changes were made, resulting in a layout that just doesn't have as much empty space everywhere. Making these changes gave me a greater sense of understanding in what SixApart was going for, but also left me frustrated, knowing that if I want these changes to be permanent, I'm going to have to modify their default structure.css whenever there is an update.
The changes made in structure.css are reflected throughout the entire site, and while the optimization was really for the editor screen, having managed to preserve vertical space helps everywhere.
Seeing how easy it was to change the structure.css got me thinking though -- custom themes for the MT Editor might not be that far away.
One of the MT developers left me feedback -- it looks like they've thought of streamlining as well and may have a solution for the compact-nav crowd.