Actionscript 3.0 :: Sprite Instance Registration Point At Runtime?

I've looked over many forums and pages to sadly learn that you can not reset/change the registration point of a movieclip dynamically. Though I am curious if or how I could go about assigning the registration point to a sprite instance I create at runtime. To give you a more concrete example of what I'm doing:

I create an instance contentWindow in my code to be animated (a 300px by 200px rect with registration in the top left corner)I create a Sprite instance imageHolder in my code and add it as a child to contentWindow I then add various images to imageHolder through a Loader object referencing an XML file. imageHolder's top left corner is oriented to contentWindow's registration point. (Leaving me with an oddly oriented imageHolder if the contentWindow's registration is centered, and will only re-size from that origin) Since imageHolder will be switching out various sized images it would be much more aesthetic to have the sprite's registration point centered. So since as3 defaults it to top left shouldn't there be a way to alter it at instantiation of the sprite? Then I could set my contentWindow's registration to center and the empty sprite would center all content loaded into it?