Hi
Assuming you are using SNP or at least planning books to read order based data (order objects in liveCache), you have key figure called "stock on hand (projected)". When you go to planning book default/start macro is triggered to show this in real time.
Same way you can create a planning book (copy of standard) and run a macro (as part of job) to calculate this on the desired level. Thereafter you can pull the data to BW for reporting. Make sure default/start macro should be switched off for this planning book.
Regards
Rahul