Masterpage Reference Another With The Same Content And Contentplaceholder Tags

I currently have three masterpages and content pages in the following hierarchy: One root-level masterpage that displays the final result. Call this "A" Two sibling pages that don't reference each other but contain all the same contentplaceholder elements, just in a different order with different <div>'s surrounding them. Both reference the root-level masterpage. Call these "B1" and "B2". Several content pages that reference one or the other sibling master pages above (not both). Call these "C1" through "C-whatever".

Cn => B1 => A
Cm => B2 => A

This hierarchy works fine. Desired Setup What I want to do is add in a new level to this hierarchy (a new master page) between the content pages and the sibling masterpages. Basically so it's like this: One root-level masterpage that displays the final result. Two sibling pages plus a third sibling. Call it B3 A new middle masterpage that dynamically 'chooses' one of the sibling masterpages. The desired behaviour is to pass through the content given by C directly to Bn without modifying it. The only thing D actively does is choose which Bn. Call this new masterpage D. Several content pages that reference the new middle master page instead of the old siblings. The challenge to this is, I'm working within the confines of a rather complex product and I cannot change the original two sibling masterpages (B1 and B2) or content pages (C) in any meaningful way.

I want:

Cn => D => B1 => A
Cm => D => B2 => A
Ck => D => B3 => A

Essentially, D should "pass through" all it's content to whichever B-level masterpage it chooses. I can't put this logic in the C-level pages. Additional Details All B-level pages have the same content/contentplaceholder tags, just ordered and styled differently. D can be as convoluted as it has to be, so long as it doesn't require modifying C or B. I'm using ASP.Net 2.0