Why doesn't HasManyThroughAssociation < AssociationCollection?

Since has_many :through associations behave more like collections after this (http://dev.rubyonrails.org/changeset/4786), is there a technical reason that the HasManyThroughAssociation can’t be an AssociationCollection?

I’m hurting for those before_add and after_add callbacks on has_many :through… can anyone give me some pointers on where to start if I wanted to make callbacks available? I’m willing to do the legwork, just wanted to know if there were any caveats I should know about.

Add callbacks to the join model. That's one of the reasons has_many
:through was made.

That was my first thought, but the attributes I want to set in my callback differ depending on which association they are being added to. There are multiple associations using the same join model with different conditions.