For this to work well it would be important that the software would have the option of dynamically tracking the read/only bit (in a similar way to how it tracks deletes).
a) if on last backup a file was writeable and subsequently becomes readonly. It would mean the server now has the latest one and on the next client backup it should treat it as though it had been deleted. b) Similarly if a file becomes writeable between backups it should be backed up once it becomes writeable as now the client has changes that are not stored on the server.

Thanks for pushing this request on.