@orbital, some idea crossed my mind today about your request. Location of "uploads" folder is regulated by config.php file. If /couch folder is accessed, it will be working with the folder specified in config. So if you can have multiple configs for different admins, you can establish separate folders for each of admin.
To illustrate the working approach - create several /couch folders and several /uploads folders - like couch1, couch2, upload1, upload2 etc. with identical configs taken from existing /couch installation. Change only the location of /uploads folder in each config. So, all your backends (mere entry points) will work with a single database but will upload images to different folder each.
Final step would be to restrict admin1 from entering backend at /couch2. Normally it is possible since the database is shared. So, here you must use htaccess password protected folders and give correct password to correct admin. So, admin1 will have password to access folder /couch1 and after that he can login to backend.
Again, your request is far from the scope of the problems Couch tries to solve (make editable and clonable HTML). So my suggestion may not be suitable in your use case, but it seems to be the only one possible workaround at this moment and foreseeable future.
EDIT: not possible. Couch keeps links to images relative to current upload directory hence links become broken.