Then do it in memory, assuming those services allow you to read the files like that. It sounds like they do based on your other comments.

The problem is, downloading files and disk management is not in your control, that part is managed by the cloud client (dropbox, google drive, et. al) transparently. The application accessing the file is just waiting akin to waiting for a disk spin up.

The filesystem is a black box for these software since they don't know where a file resides. If you want control, you need to talk with every party, incl. the cloud provider, a-la rclone style.