Does Drupal's read-only mode module work for Civicrm too?
In short - no.  Moreover, if it did, it would be dangerous, because the Drupal module can't know which database writes are part of the legitimate upgrade and which aren't.

In my experience, only upgrades that add/remove indices take any appreciable amount of time.  Is it possible that more frequent upgrades will result in shorter downtimes for your use case?