I got that most useful answer below from Steve Tuckner.
I want to share it with all of you, so that it will show up
in the archives of the mailinglist.
-------
Armin,

I tried to post this to the list but failed (it never appeared -- twice). So
here goes:

As far as I know, there is no way to do this with any VRuby object. In my
project I adopted the convention of using exinit as my initializer. So for
example, to create a yellow button, I might call with the following code:

addControl(VRColoredButton, 'lab1', "ta", 100,90,200,100).exinit(YELLOW)

class VRColoredButton < VRButton
    def self_created
        # called that the button is created
    end

    def exinit(color)
        # set the color here
    end
end

I need to release my VRTools kit that has some enhancements when working
with vruby.

Steve Tuckner