Actually, I'm not sure that a call is failing.  When I increase the width of the wxChoice object in compiler_toolbar_16x16.xrc, the toolbar extends accordingly.  Also, when I move the wxChoice between the other "tools", it takes up the correct amount of space.  So, the object is apparently "there" but just not visible.
UPDATE: I can replicate the behavior in my sample by calling wxChoice_instance->Show(false).
So, how could visibility get set to false?