I think there are arguments for simply doing this kind of thing in MS Office's SmartArt.

(And perhaps copy-pasting into OG, if need be)

Bloat and bureaucracy apart, there are still things which are simply done better on the scale of Redmond, and, like Excel, the SmartArt layout engine is one of them ...

A key strength, apart from the really quite impressive range of diagram types that it will convert text outlines into, is the automatic resizing of fonts. As you type more into a text node, the font size changes intelligently, so that all nodes at the same level have the same font size (the largest that is feasible), and parent nodes never have a smaller size than their children.

Omnigraffle would never be able to do this, even with the help of script ...

(Some scripting environments -Powerpoint and Indesign come to mind, expose the surface area occupied by a piece of script at a certain font size, so that you can automatically calculate the largest point size that will fit, but the Omnigraffle Applescript library gives no access to this. It doesn't even give direct access to the outline hierarchy - you have to derive it indirectly from connectors, which wouldn't be an option with many types of diagram, including block hierarchies like these).

In short, MS Office is far from perfect, but it's probably the thing to use for this ...

(OG is great for flexible formatting and diagrammatic sketching, but MS SmartArt is hard to beat for anything based on nested text).