Per group registration code feature

I think the registration code is a nice feature, but I don't think this have to be site level. All groups shouldn't share registration codes, and codes should be entered at group level (not from group directory, when you enter a registration code, you don't even know for each group it will apply before you actually entered it, this is somehow frustrating for users).

Something even more frustrating with this model is because your groups share the registration codes database, and it can't have any duplicates, it creates a security issue: if a group admin tries to add a registration code for his group, and the saving fails, then he knows that this code works for another group and then he can subscribe to it without any security checks.

I don't *need* this feature, but it would be great, for security, but also at end user point of view.