Hi Menno, I'm glad that you are liking MoI!

For cross sections, you can generate cross-section curves by drawing in a line and using Consruct > Curve > Project to project the line onto the object.

The arrangement of lines would look like this example here:
http://moi3d.com/forum/index.php?webtag=MOI&msg=4295.3

Then you can run Construct > Planar on those curves to generate a planar surface instead of just curves, or another alternative to generate planes more directly is to draw in the same lines and use them with Construct > Boolean > Intersection - internally to the boolean the lines will extrude out to planes and the boolean intersection of a plane with a solid will be a section plane.

Hope that helps!

- Michael