Apologies are in order.

Having taken a look at "Dlg_SelectMaterials.xml" and doing a search for widgetProcID="31", the other places that a scrollfield turns up are "walkDesign.xml" (another with a vertical scroll bar) and "scenePalette.xml" (a horizontal scroll bar).

The way the walk designer uses it suggests that it acts like a generic container, i.e. all the widget elements enclosed by the scrollfield widget element are the contents that appear when the dialog is shown.
In the case of the walk designer, it's a fixed list of other widgets, so generating something on the fly should be possible by using some PzrXMLTag objects to generate the 3 slider grove widgets and the scrollfield.

Will have to investigate if it's possible (and useful) to set a callback on them.