The extra div and css is plenty to do all the formating you want with signatures.
I thought so too, but not quite. The reply text is inside a <ul> element inside the reply <td> element, so that the text shifts a bit to the right. The sig divs and text are also inside that <ul>. That prevents some of the potential formatting because the sig block doesn't extend horizontally over the entire reply area. For example, you can't create a sig box that matches the subject box at the top of the reply node. The sig box will only extend left to the left margin of the text, not to the left border of the reply node. And because the sig is in the same <td> as the text, it will only extend right to the box with the "[reply]" link, not all the way to the right of the node.