You’re right. Even the MBS property can’t be assigned. I don’t use iCloud for storing files either, so I didn’t actually tried to use that.
I’ve looked at other properties given by the plugin and setting the “keep only locally” flag doesn’t seem to exist there. Perhaps it’s just not settable.
However, there are 2 interesting properties extending the FolderItem class:
IsDownloadedMBS: whether there’s a local copy of the file.
IsUploadedMBS: whether there’s a remote copy of the file.
Now, it looks like when a file exists only remotely, Xojo’s “exists” property returns “false”; but it’s still a valid (though “non-existent”) folderitem, so I guess these two methods above would permit to know whether the file actually exists (locally, remotely, both, or not).