This is write about my experience: https://forums.macrumors.com/posts/15223182/
The link goes to post 249 in another similar thread.
Basically you should take the SIUResources.pkg from your Lion installer and include that together with all the other packages in the folder named Packages. Nothing else was done for me to get to where I describe. Well, it's not the only thing - I did follow the instructions and got what I needed. Now I have a Mountain Lion partition that just needs to boot into the actual installer.
mine doesnt tell me what is missing.... it just says its missing some required components...