If it was a site with "heavy write traffic" wouldn't the database already be stale anyway while the dump was being taken and restored? Zero downtime maybe not so hard. Zero write loss and zero read-only time is harder.
If it was a website with heavy traffic mysqldump would possibly take hours to complete and lock your tables for so long it affects requests. Even xtrabackup isn't suitable to run on a master node and it's common to have a dedicated slave for backup purpose.