How do you prevent the flat list of members that users see from alternate hierarchies?

Summary:

When users get access to members via alternate hierarchies that they don't via the primary hierarchy, they see the members that are in the alternate via a flat list. How can we stop them from seeing this?

Content (please ensure you mask any confidential information):

To deal with reorganizations in our annual planning process we have a current year organization hierarchy as well as a Yr1 alternate hierarchy. We use the current year hierarchy for almost all scenarios except for Plan. Plan ONLY uses the Yr1 hierarchy as it is for the next year. We don't change up the organizational security in general as if a user is in a "Group A" security group, we want them to have access to "Group A" for most scenarios and "Yr1_Group A" for Plan only. Unfortunately, if a person has access to members via