There is a very neat workaround for this.
Take a Morph and draw a big rectangle (bigger than your mesh) in the floor plan.
Then set the morph at the right height where you want to cut.
Now SEO the morph and the mesh, namely with the not often used "Intersection" method where the
mesh acts as Operator
(!) and the morph as Target.
Now, if your morph has a cover fill, it will only visible exactly where you need it
😉