1.06 will have a number of new script functions that will give improved control over custom GUIs.

Two of the new script functions are:

SetGUIProgressBarPosition();

This script function will let you set the position of a progress bar (Between 0.0 for empty and 1.0 for full).

The other new function is:

SetGUITexture();

This script function will let you set the texture for a UIIcon, a UIFrame's fill texture, or a Button's Base Frame's fill texture.

Both script functions require the UIScene to be scriptloadable like all the other GUI related scripting functions.