I have extruded a simple closed polyline curve and capped it. Can I fillet the top and bottom edges where the surfaces meet? I know how to do it in Rhino, I'm just wondering if I can do this with GH as I'm enjoying being able to live-view various proportions and dimensions and iterations.
filleting of surfaces is not exposed as a function in the Rhino SDK. You will not be able to do this in Grasshopper, a custom DotNET plugin or Rhinoscript.
I do now know when filleting will be exposed, it might be quite a while though.