Hi Chris,
I have faced similar type requirement for one of the Client,
System Includes Actiivity Cost also in Change in Stock Account (Which is posted at the time of Goods Receipt), Create the Seperate Profit Centre for Activity Type Cost which shows the Activity Cost seperately in Profit Centre Accounting.
It's not advisable to use any Exit or any Development to Split the Cost related to Production.
According to My Knowledge this is only for Reporting Purpose, Just cross check with Business.
Regards
Srinu