Yes, because the problem is that, although the selection box moves in a patterned way, there are "gaps" in its visibility. So, let's say you're in Firefox viewing a Google search result page (for example. I think this tool should be awesome for Windows, globally--and not just for Firefox). You're using alt to navigate the links and browser buttons, etc. You can see the trail of the little contiguous jumps in selection/focus, but then you'll lose track of the selection/focus as you're holding down alt, and you can't see where it went. (In one instance, the focus will jump to the entire browser window and you don't think to look at edges of the screen. It took me forever to see that).
All my babble boils down to this: the pattern of the movement of the selection box can be very complex in various navigation circumstances. If you can't see it at a certain time, you can't grasp the pattern. If you can't grasp the pattern, then you'd need a mouse!
-MC Spanky McGee
PS. I love the project you guys have going.
PPS. I already tried to find the software I'm imagining here (the closest I came is a Firefox add-on called Highlight Focus. But it doesn't work for Windows globally and it doesn't highlight all that well) With all the sites I've seen on ditching the mouse, you might become heroes with this project (not that you weren't already).