You can make the text3d object a child of the billboard.
Code:
import viz
viz.go()
billboard = viz.add('billboard.3ds', pos = [0,0,5])
#create text object and make it a child of the billboard
text = viz.addText( 'message', billboard )
text.setScale(.1,.1,.1)
text.alignment(viz.TEXT_CENTER_CENTER)
text.color(viz.BLACK)
text.setPosition(0,1.7,-.05)
#press space bar to change message
vizact.onkeydown(' ', text.message, "new message")
Depending on what you want to do with this billboard it might be better to create textures with different text in them and swap the textures on the billboard
By the way, we recommend exporting 3ds models into Vizards native file format (.ive/.osg). It supports baked lighting and other advanced features.
You can download the exporter for Max from our downloads page.