Sure you can. Unimproved value of the land is simply the value of the lot as a whole minus the value of building that sits atop (well, improvements aren't always buildings, but that's a simple example). You can approximate it by comparing it against the transaction prices of nearby lots that aren't improved, or by taking the construction cost out of the purchase price of the lot.

> is popular with politicians

Land value tax in general isn't that popular around the world, and Henry George's version of it, where all tax is replaced with just a land value tax, even less so. As far as I'm aware, there is literally no jurisdiction in the world whose sole tax revenue is from LVT. Economists and idealists like land value tax; politicians, who derive a lot of power from the wealthy landowning class (who in turn would be the most hurt by LVT), not so much.

> The bigger issue is with vacant properties being used as cash stores.

Land value tax would be an excellent way to curb real estate speculation.