Thanks. I've actually spent the day picking apart some of the simpler sidebar workflows to see what makes them tick, and having some success figuring out what I want to do and how I want to implement it. What I want the sidebar to do has evolved a bit since I asked my original question. Now, having discovered that I can load UI elements into the sidebar, what I want to have is several textview windows in that sidebar, into which I'd be able load the contents of different files (which I'd choose via selecting from a list). I've managed to make it work via actions in Custom UI, but making it work in a sidebar is going to be just a bit more complicated. Still, it seems very doable.