Yes, what you are suggesting would work perfectly. Regarding the concern about allowing a user to "opt out", I don't think this should be an option. The intent is to design and incorporate this feature for "unique groups" where participation is mandatory. However, there are always exceptions to every rule. For example, suppose an indvidual becomes very ill for several months or travels overseas on an extended vacation then there would be the need to disable their notifications.
I am not fully up to speed on how everything functions and integrates but here's something that may provide an option for an individual to opt out. In the section "User Profile - Notifications and Email", the last option is: For topics and boards I've requested notification on, notify me of: "Nothing at all". Perhaps this could be an overide to disable any notifications, can you confirm if this would work?