Andrew, don't you know if the picture from the A7s matches the one from the D750, someway?
I was thinking that using for istance the Sony Cine2 PP6, it is less versatile then the S-log2 PP7 but it doesn't need to be graded so much. Maybe it behaves closer to the D750 flat profile then the S-Log.
In this way A7s and D750 can work in the same set as camera A and camera B without many postproduction problems.