It's hard to do, for sure, the way the TD layout works. In HTML, you could have a table row and specify each cell's size with percentages. This should size appropriately and allow the middle column to center regardless of resolution.
With the TD, you specify the size of the items within each "slot" in a row (I'd use the word "cell" but that's the same word that names an item in a TD layout). In a sense, each cell in a row isn't aware of the other cells, so you can't specify a percentage width in the same way.
Usually you'd have to hard code the widths of the outer cells so that the inner cell is centered - but it is screen resolution dependent. Change the resolution and you have to modify the layout to make it center appropriately.
In 3.2, there is an option for an HTML layout, in which the entire layout is specified using HTML (there are no TD Rows, Columns, or Cells). You just embed layout tokens directly in your HTML. This will make it much more flexible, if you're an HTML-coding-type-person. It will definitely allow what you're trying to do.
Not much help now, I'm sure. Sorry!