Ruby/TK Iwidget combobox sub-classing

Hello,

I would like to sub-class the Ruby/Tk iwidget combobox class.
I want to be able to post several commands when the "return" key is 
pressed on the entry of my objects but I also want to preserve the 
default combobox insertion functionality (new entries are automatically 
inserted in the scrolled list)

The class code could be like ...

class MyCombobox << Tk::Iwidgets::Combobox
   def initialize(parent, *args)
      super(parent, *args)
      ...
      @commands = 
[]                                                         # array of 
commands
      @commands << cget(:command) if cget(:command) != 
""                    # the first command launched will be the default 
command
      configure :command=>proc {val=get;@commands.each {|c| 
c.call(val)}}    # launch all commands when a new entry
      ...
   end
   attr_reader :commands
   ...
end

... and the usage ...

cbb = MyCombobox.new...
cbb.commands << proc {|val| puts "new entry #{val}"}

The problem is that this code is not working because the default 
combobox command is a Tcl command (not a ruby proc).
So, my questions:
- is there a simple (and portable) way to launch Tcl commands from ruby ?
- is there a more elegant way to add a list of ruby commands while 
preserving default Tcl/Tk megawidget functionalities ?
- is there any documentation/help on Ruby/Tk and ruby Tk extensions ? 
(without having to understand the full code)

Thanks a lot for your help and regards,
Lionel Maiaux