Then you should not do Plant Stock, you should do Project Stock. That way all of the inventory is kept separate.
Or, if you use plant stock you can look at the pegging to see how the supply and demands are linked. This is a soft link and could change every time MRP runs.
I will say if makes no sense to create plant stock reservations to run MRP and purchase material and then transfer material to project stock, and then issue it a project. This is a lot of manual work with no benefit.