Your idea will work. The following code adds the info box only to a transparent window on top:
Code:
import viz
viz.go()
gallery = viz.add('gallery.ive')
import vizinfo
info = vizinfo.add('Info box in transparent window')
info.translate(0.05,0.95)
info.alignment(vizinfo.UPPER_LEFT)
#don't add info box to MainWindow
viz.MainWindow.visible(0,viz.SCREEN)
#add a subwindow
subWindow = viz.addWindow()
subWindow.setPosition([0,1])
#dont' add info box here
subWindow.visible(0,viz.SCREEN)
#add a clearWindow that the info box appears in
clearWindow = viz.addWindow()
clearWindow.setClearMask(viz.GL_STENCIL_BUFFER_BIT)
clearWindow.setPosition([0,1])
clearWindow.setSize([1,1])