Well, it is almost possible. You can create a window that appears under each network that could have that information. It's not all grouped together, but it's almost the same.

Code:
network_1
|- Channel
  |- #channel_1
  |- #channel_2
|-Window
  |- channel_1
  |- channel_2
network_2
|- Channel
  |- #channel_1
  |- #channel_2
|-Window
  |- channel_1
  |- channel_2


Still, more scripting options for the treebar isn't a bad thing. Or even scriptable control of both treebar and switchbar to move channels/windows around, add or remove divider bars, etc.