I've not had much call in developing Python AddIns but what you were asking is something I would have approached in VB .net using an extension with an event firing on something specific.

If you look at the help file you can create a *Python add-in application extension*. This can hook into various events, an obvious one is on openDocument() or possibly contentsChanged(); you could get the combobox to refresh on that.

As I've never developed a Python AddIn what I am unsure about and hopefully someone can confirm is how one hooks into the combobox in your python addIn toolbar from the Application extension to update it.