Hi Max, there is not any support in the geometry library that MoI uses for doing exactly that kind of vertex-only fillet automatically so it's not very easy for me to include it as a built in automatic type tool in MoI anytime soon at least.

I guess for something like the bolt head that you are showing you would also not need just any totally symmetrical chamfer but also want to choose something about the angle involved, that makes it fairly complex overall.

Is there another CAD program that includes that as a built in tool so that I could check it out and see how they've done the UI for it?

For now and for quite a while at least you will need to use the method described in that link Martin posted above:
http://moi3d.com/forum/index.php?webtag=MOI&msg=5062.8

The benefit of that method is that you get a lot of control over exactly how you want the chamfer formed, you can control the angle of the cutting shape, or do a sphere as the cutting surface or whatever.

- Michael