Getting storage and transfer stats for a subdirectory?

I’m working on an app to streamline sharing assets online (images, video, etc.). Right now, the user has to have their own S3 account.

But I’m thinking about offering a version that handles the service aspect, too, essentially re-selling my Spaces storage. Is there any kind of built-in support for determining how much storage and transfer a particular user is using across a subset of objects? I hope to have thousands of users, so I can’t really create a bucket for each one.

I was thinking a subdirectory for each user would let me keep their content separate from others, making it easier to measure. Is there a single call to get the recursive size of a directory, and/or bytes transferred to/from it?

Or maybe there’s a way to tag each object they upload with an identifier, and then I can query Spaces based on that tag? Like, return all keys with tag “12345”? I want to be able to impose quotas on my users, and hide or delete objects if they stop paying.