If each such 'structure' could be put in a separate device partition, either directly or probably handier using symbolic links to isolate it from where it normally lives, then Filesys:Df would still do the job more efficiently than having to recalculate the (f)statvfs yourself.

Update: The way my own hosting supplier does it is to have a separate partition per client, symbolically link the top directory of each website structure as a subdirectory of where the apache server is installed and alias each website to that directory in the httpd.conf. They have a different location for all the webmail though, just because that has a different tariff per MB.

I imagine there is a downside that they have to automate partition allocation and it is hard for customers to give up disk space they have requested because extending partitions is significantly easier than recycling part of an allocated partition, even more so if this has to be automated.