Hi NightCabbage,

> I think the problem is that, to my knowledge, these
> features aren't documented?

Being able to fillet corners of a single joined curve is documented in the help file under Fillet:
http://moi3d.com/2.0/docs/moi_command_reference7.htm#fillet

"Selecting a single curve that has corners in it (for example, a rectangle curve) will allow filleting of some or all of those corners."

Also one of the example images shows that kind of filleting with selecting corner vertices...

- Michael