For collaborative editing to list a set of files that can be edited, a level of information about read and write access to files/folders is needed.
An API function to list this sort of metadata in a way that is optimized for the API client information ("read-only" vs. "read-write" could be sufficient, rather than "lrs" and other such IMAP rights) would be useful in determining the level of access.
This new functionality could then also be used to reduce the folders in a "Save to cloud..." type of interface, to only list folders a user can actually write to.