Not an answer, only a workaround
Okay, one lackluster workaround that i've found elsewhere is to restore the QuickLaunch Bar. I've done this in the meantime, but it doesn't actually do what i want -- so i'm still looking for a real way to change the taskbar icon spacing. I'm posting this answer with the thought that some may find that it is good enough.
The addition of a QuickLaunch Bar looks it look something like this:
So what i'm doing is putting the older-style apps (Visual Studio 6, 2005, and 2008, UltraEdit, IrfanView) down in the QuickLaunch, and leaving the W7-aware apps in the taskbar, so i the taskbar integration where implemented. Zune, for example, shows player controls when you hover over the icon and Smart DJ items, etc. in the popup. The downside, as you can see is that some programs (like UltraEdit) show up twice now; once in the QuickLaunch and once in the taskbar.
The benefits to this approach: when not running, the icons are handy and aren't triple-spaced.
But the downside to this approach: when running, the icons are doubled, taking even more space than before; visually it's slightly confusing; nifty taskbar features like Recent Items are not available.
So, this is one option, but doesn't really answer my question.