Well in turns out that the OS X keystrokes for this action are the same in Mac OS 9 as follows:
1) Open Finder window in List view
2) Highlight any number of folders (or all of them)
3) Hold down the <Option> and<Apple> keys and press the <Right Arrow> key at the same time
This will toggle the Disclosure triangles to their "open" position.
To toggle the Disclosure triangles to their "closed" position use the same technique but press the <Left Arrow> instead.