How to index one folder on a (huge) smb shared drive for Spotlight?
I'm not sure you can index a single folder in quite that way. Indexing starts at drive level…

But how about the other way round?
Tell it to index the drive, then add everything else to Private?
Exclusive rather than inclusive method.

You can do that at the server by adding a file called .metadata_never_index at the top level of each other directory, rather than have to do it at each user's machine from the Spotlight control panel.