It might be useful to Sandcastle users if automation tools were able to report the progress of Build Assembler. An event on the BuildAssembler class would be great, like ProgressChanged. Or you could add a new constant to the MessageLevel enumeration named, "Progress".
The original feature request is here:
http://forums.microsoft.com/MSDN/ShowPost.aspx?PostID=2132079&SiteID=1