Hey @timbutler,
Apologize for the delay. As you probably read access to arbitrary files on the system is not typically a justification for requesting classic confinement.
For user files, I see you could plug home with the read:all attribute to get read access to non-hidden files in the home directories of all users as traditional file permissions allow.
For system-files you are correct that you need to specify the directories your snap needs to access, as it is typically used for providing read-only access to system configuration directories to snaps that are the clear owner of the directory in order to support importing from existing applications.
snap connect is used to connect interfaces, and there is no other snap command for a user to specify access to locations. Instead, accesses are granted through declarations on the store side.
Feel free to add further questions if needed.
Thanks!