Hi okapi, yes I plan to do some work for mitered corners on sweep, it will probably be the last part of work that goes into v2.

At the moment to finish that shape you would need to do the sweep which will generate segments like this (here I removed the caps):



Then select each ending edge and extrude it with caps off, to produce an extension like this:



Then select both extensions:



And then do a mutual Trim - this is when you select both objects to be trimmed and when it asks you to select cutting objects, just right-click or push Done then to signal that each will be cut and also cut the other. Discard the extra pieces to get this:



Also you do a similar trim to the main pieces to discard their overlapping internal pieces, and then you can Join the results together.

That's how you can currently get a mitered corner, I do expect to update sweep to handle this automatically for you but that is not ready in the current version of MoI yet.

- Michael