When they go into an oven they are on a particular sport on a rack. When they come out they will still be in the same spot. 

That is your tracking method through the oven: track the rack the item is on and where it is on the rack. This also lets you collect data on which spots on the rack don't do as well as others.

However most bulk abrasive treatments for small machined parts is putting a bunch of parts into a container along with abrasive elements and then the container is agitated. 

If the part is larger then you would be using additional machining to run sanding/grinding disks along it (or hand sanding). Again here you can track the position of the part instead of the part itself. Up until you can add the serial number.