It is possible to use the zone system for slide film in a limited way, but it's reversed--expose for the highlights and develop for the shadows. Or it's the same, if you think that you need to expose for the thinnest part of the film, and develop for the densest part of the film.
In other words, if you know that the contrast range is within the range of the film, like in the studio or flat natural light, you can often just take an incident reading or gray card reading and come out okay.
If you are in contrasty lighting conditions or have a contrasty subject, you might want to place the brightest highlight that you want to show detail about 1-1/3 or 1-1/2 stops over neutral gray (i.e., around zone VI-1/2), and then you can let the shadows fall where they may, or if you want to calibrate it, you can make a development test to see how development time changes the shadow placement. If you use a lab, see what happens if you ask them to push or pull while keeping exposure constant. You might find that the lab's "1.5 stop pull" for instance, is a -1 development in ZS terms.
Still, you're not going to get the kind of compression with slide film that you can get with B&W neg film, so a better solution is to use grad filters where possible and when you need them.
If you're in flat light, you can boost contrast by increasing development time or asking the lab to push. Practically, you don't have as much control as you would with negative film, so the way I usually apply this principle is just to push one stop in flat light, and if it's really flat, I may switch to a more saturated film.