It's still not clear what the problem is. Are you getting a Python exception when trying to set an attribute? If so, what is the error? Can you post some sample code that reproduces the error?
The following code works correctly for me:
Code:
model = viz.addChild('gallery.osgb')
group = model.insertGroupBelow('gallery')
group.myattribute = 'some value'
print group.myattribute