Link has already identified the "free line cut" method you ask for - as close as it can be in ArchiCAD:
to use the Section line tool in 3D via the special menu - but this doesn't allow vertical definitions like the SEO knockout method we have just discussed - ooops. it does....
You can also define your visualization area with a complex marquee, selecting only the storey you want to visualize.... and preserving the marquee path by using an outline on a special layer....
each of these are only one or two steps...... but feel free to add your request to the official wishlist. If you were to upgrade to Version 8.1, I am certain that you would find these alternatives adequate.
Dwight Atkinson