I'm not sure you have that level of control in a multi-box. See attached for a cheat though! I took an image of the closed multibox and but it as the background of a textbox. The show condition of the textbox is
GetPossibleCount(Field2)=1 AND GetSelectedCount(Field2)=0
So it will only show and cover up the value "A" when there is only a single possible value and also when you haven't actually selected it.
Hope this helps,
Multibox Display.qvw 147.0 K