You could add multiple text objects to a panel and change them individually:
Code:
import viz
import vizinfo
import vizact
viz.go()
panel = vizinfo.InfoPanel('Message panel')
panel.addSeparator()
text1 = panel.addItem(viz.addText('message 1'))
text2 = panel.addItem(viz.addText('message 2'))
text3 = panel.addItem(viz.addText('message 3'))
vizact.onkeydown(' ',text2.message,'new message')