How do I add a slider to an AppIndicator?
I've asked the question on the ayatana mailing list and received an answer from Ted Gould:
Unfortunately that's not possible today. It's been our intention to support custom menu items in dbusmenu so that you could do anything that your app needed for your application indicator (including using IDO) but I haven't found the time yet to complete it :-/
So the answer can be closed.
EDIT: In case anyone has further interest in the project they could try contacting Ted to see how they could help out.