Thomphl,
So sorry that I overlooked RFQ process in place before PO generation and more over the solution I suggested above doesn’t really work as once you create PO the Project ID field under reference becomes disabled to enter. Microsoft quotes in R3 it is possible but we need to confirm on the same.
Now for your scenario, here is what you can try doing:
For every Item requirement you enter against a project, there will be a sales order created with type Item requirement. So while you run through master planning, it creates the planned order with pegging relation to this sales order. Here you need to take care of Item coverage planning to take care of consolidation requirement.
Now, as you create a RFQ for the goods create multiple lines with different Project IDs in line details. To do this, use either Addline or Copy line functionality in RFQ. Once you get the reply from Vendors enter them and accept the lines, which will create purchase order with different Project ID lines. This way you can have reference of Project against PO lines. Try this out and let me know.