It is possible to add users to a workspace as a Workspace Reviewer that gives the user Read Only rights. Is there any way to add users to a workspace as Write Only participants? In this use case participants would be able to upload content but not download content. The idea is to use a workspace as a dropbox for multiple participants where each Write Only participant cannot read the other Write Only participant's content.
Did you check the default workspace participant roles, if not have a look at it. User who got ability to upload content will be able to download content as well. I'm not aware of any ways by which we can restrict this.
ACL's can be used to selectively hide folders in a workspace. Have a look at the following documentation for setting folder level ACL's