This information is available in a CSV report that can be exported from the DreamObjects Usage Tab. Under "Usage Report," choose DreamObjects Daily Usage. You can see all users in one report or export one report for all users.
Since you're looking for a way to do this programatically, you could try tool s3cmd and use the du option. It will step through each bucket and calculate the storage and output in bytes. In order to do this for multiple users, you'd just have to pass the appropriate key pairs for each user.
Here's the default configuration file needed for DreamObjects.
$ cat .s3cfg
access_key = Your_DreamObjects_Access_Key
secret_key = Your_DreamObjects_Secret_Key
host_base = objects.dreamhost.com
host_bucket = %(bucket)s.objects.dreamhost.com
enable_multipart = True
multipart_chunk_size_mb = 15
use_https = True
And here's more info on s3cmd - http://s3tools.org/s3cmd