Hi Jeremy,
You can write logic in program RV45C905 form DATEN_KOPIEREN_905. Add enhancement implementation for the document creating with reference to invoice inside and change the plant which you want. It might work for invoice.
If you are creating order with reference to order keep code in USEREXIT_MOVE_FIELD_TO_VBAP of MV45AFZZ.
it might work.
Award points if it is helpful.
Thanks,
Santosh