It's possible, but you need to program your own label on GDL, that has such functionality.
If you want, you can check how it works in the library I was developing for Jerusalem architectural company (there some Hebrew in the template mixed with English).
If you like my object - I can extract it from there and with small modification make it work for you.
I suppose with GDL you can actually grab the top layer thickness?
I was going to suggest creating a property for the level and one for the thickness of the finish (which won't be linked to the composite). You can grab the elevation and substract the set property for thickness to return the correct elevation value. Instead of using level dimension you would use a label that shows the property.
Added bonus: you can show the elevation property in schedules.
I have something similar set up for pile foundations with elevation to sea level etc
With GDL you can get thickness of each skin in composite structure of slab. Also skin property - finish or core.
I wouldn't recommend to mess with properties. Because to make something similar with properties - you need to set property expression, that means for expression you will need to write the script anyway - and at the end you will not get so elegant and simple label, as you could do with GDL. In GDL you also can have any sort of marker to follow national CAD standards.