I haven't tried the process of launching a macro from an addin. But I have accomplished the same thing (loading placeholder model, then closing other model) using this method http://thebuildingcoder.typepad.com/blog/2010/10/closing-the-active-document-and-why-not-to.html
Steps:
1) Open placeholder model document
2) Close the original model document
3) Activate the new thread to send the Ctrl-F4 to the placeholder model (assuming no other models open)