Support

Such a feature was never implemented because it makes no sense if you think about it. Each email has a URL to launch the one-click update. That link logs you in as a Super Administrator, in fact the exact SA user the email address belongs to. That's why the one-click update link in each email is different than any other. If the email was sent to a different email address we'd have two options:
1. Not include the one-click link. In this case it's equivalent to subscribing to our RSS feed to find out when we release a new version of our software, i.e. a useless feature.
2. Include a one-click update link logging you in as one of the Super Administrators. This is s huge security hole which is called "privilege escalation" as it allows someone who normally has no SA access to become Super Administrator.
That's why the feature to select an arbitrary email address was never implemented and never will be :)

Normally, all Super Administrator should be receiving the email. Do they all belong to the user group #8 (the default Super Administrators group which ships with Joomla!)?

This is incorrect. If a user belongs to the Super Users groups (ID = 8 <-- that's the condition we are searching) and his account has "Receive System emails" set to Yes, then he will receive an email. If he doesn't belong to group with ID=8 or he has "Receive System emails" set to No then he will not receive an email. Maybe you need to check the ""Receive System emails" setting for the user in question?