The Zone System is a method to examine the scene in front of you, analyse its brightness range, decide what kind of brightness range you want in your final print (visualise the final print), and then chose an exposure, and then a development, that - given the film you use, the developer you use, the developing process, the paper and the tests you made with them - you know will give you a negative that, when printed*, will give you that final print just as you visualised it.
So the zone system asks you to make, for each frame, a certain exposure and development choice finalised to a certain desired result, especially regarding contrast.
All this makes no sense when using slides, or when printing from scanned film. You have the basic exposure problem (how to make the brightness range fit inside the dynamic range of your film) and that's all. Once your slide is properly exposed, a good dedicated film scanner will capture all its dynamic range, and in postprocessing you will obtain the contrast that you want using what Photoshop calls "Levels" and "Curves". You only need to expose your film "properly" and to have a capable scanner, you don't need to make exposure and developing decisions frame by frame. Your question will be answered by the many threads about exposure here on APUG.
A small variant might be that, if you really think that your slide will see no other use than scanning, then you might choose to underexpose them a bit, as a good scanner will be able to dig in the shadows and you'll be able to recover shadows but - as with digital - a clipped highlight is detail lost forever. I personally don't belong to that school, though, and I try to expose normally "for the highlights" figuring where they will fall on the slide, as scanning quality is influenced by density, and a darker slide is recoverable, but the result is not optimal, so I try to exploit all the (limited) curve I have on the highlights as well.
I understand I did not answer your original question - a workaround for a densitometer - but I think I somehow gave an answer to the underlying assumption.
* When printed on a paper of a certain contrast. This original version of the zone system is somehow made redundant by the introduction of variable contrast papers as far as I can read, but I'm no printer (yet).