[FR] Manage and restrict assets by site or site group within an assets volume on a Multi site system

[brionwolf]

Our team supports a few of Multi site Craft instances, with 3 to 10 sites per system, where each site on a system uses the same styles, and structure, but we need to keep content authors, their content, and (preferably) assets restricted to a specific site. An editor with access only to site Foo, should not be able to access the content from site Bar, and vice versa. Setting translation methods on sections, entries, and their related fields works as expected, including relational entries fields. But we're getting tripped up on managing/restricting assets between sites.

While the fields on those assets are translatable, we're worried less about the content on an asset so much as we're concerned with the asset itself. I found the suggestion in #3586 to make assets volumes for each site and using permissions to show/hide them on a particular field, but our systems are setup with 3 to 4 volumes (multiplied by the number of sites on a system). We also tried a version of this answer on Stack Overflow), but then we have to remove a user's access to create folders, and they could still navigate to the assets control panel to cause content issues.

I know this isn't easy since, as Brandon says in #8539 "there is currently a direct correlation between assets and the physical files on the server". We would really love a solution for handling assets separately on a Multi site system. Ideally that would mean making assets available to one site or another like we can with structure and channel entries, but we're love to hear other ideas.

Thanks for the time and help!