I understand you are trying to develop a Z- report through a Function module if possible - am I right? I am however, missing the connection between spl procurement key and CS15...folks seem to be getting confused between how to achieve the scenario of production of component in plant 1000 while product is made in plant 1100 & how to get a report where that scenario already is configured!
I would say, just go about documenting your functional spec based on the 'product structure' report...("implosion" like report). Do let me know if you are unable to find the product structure report from google.com.
In the mean time, I promise to search for some function modules which might help for this scenario...
good luck..