Filter Group Selection When Used for User-Modifiable Enrollment Attribute

Current functionality supports ability for users, during self enrollment, (or admins, during sponsored enrollment) to select an optional group membership via enrollment attr 'Group Member'. However, the need is to constrain the drop-down list of groups to a set of defined values. Obviously, we would want to prevent the 'admin' groups, but we would want to further constrain the list dependent on scenario. The best solution would be to define the list of acceptable groups in the enrollment attribute, e.g., allow group A, D, Q, or nothing. Going further, in addition to explicitly defined groups, allow multi-select (more than one group if allowed).