Thanks Ronghua. It seems my client's scenario does not deal with Special stock(sales or project stock). I found a solution though for what exactly i was looking for.
Pretty simple. I changed LIPS-PRCTR during Delivery Creation in
User Exit: MV50AFZ1
FORM userexit_move_field_to_lips.
Which did not get changed back to material master PC when doing PGI.
Note: My ultimate requirement for doing at PGI level was to forward the PC values substituted at PGI level to Accounting Document. But it got changed for all G/L Accounts which was not expected. My business requirement is still left unsolved.
Closing this thread.