>Make sure that all of the masters are inherited from mmMasterPage and not from the standard master page. You would need to add some Mm using statements but they need to all be subclassed from mmMasterPage the same.

The IASLeftNav was just a MasterPage, not mmMasterPage, but that still hasn't worked and I've tested it by setting a breakpoint in all three page_loads. The debugger then jumps straight from the content page's page_load to the OuterMaster, skipping over tha IASLeftNav completely.