Hi all,
I don't really understand the problematic of creating the gui for Octave, but in my opinion a great inspiration can be taken form the RStudio project (http://rstudio.org/). The RStudio is a gui front-end for the R-language and it contains an interacting editor (code completion, F1 key = jump to a help window, etc.), command window, workspace etc. The project seems to be very mature and it is also available for different platforms including Windows.