Thanks to whoever added this, it's fantastic!
I have one request though: can you please combine the four envelope paths into one four-sided envelope? It will be much easier to edit this way. I understand that this may introduce the ambiguity as to where one side of the envelope ends and another starts, because the envelope sides may be more than 2 nodes long. But I have a rough idea on how to handle this: Imagine a line that goes at 45 degrees from SW to NE, and imagine that that line moves from infinity in the SE direction. The first node of the envelope path that it touches will be considered the top of the left side and the left end of the top side. Same for finding the 3 other corners. I think it will be relatively easy to implement - and much easier to use than the four separate sides.
Thanks again, and by the way please add a description of the new feature to http://wiki.inkscape.org/wiki/index.php/ReleaseNotes047