Hi Niko - MoI does not yet perform mitered corners on a sweep which is what you want for this case.

I am planning on working on in for v2, probably right near the end.

Currently that one will be easier to do in Rhino since it does the mitering, which involves an automatic extension on each section and trimming. For MoI currently you would need to use an alternate approach, such as this:
http://moi3d.com/forum/index.php?webtag=MOI&msg=428.12

Or like Burr mentions some manual trimming of each section can also be used currently.

- Michael