If it would be the same number for each reservation, I don't understand why your SM number can't fulfill that same purpose. I never use the PNR/Confirmation number. No matter where I am interfacing with DL I just give them my SM number and have them look up my individual reservation. Seems that it is the exact same thing.
Only difference would be accessing them from another airline, but even a "reference" PNR probably wouldn't work across IT platforms anyway.