I used a couple of R8 spindle machines for years without any key in the spindle. Your source of information is correct the key stops the collet turning while you tighten the bolt it does nothing to held the drive.
Is it your idea to replace the drawbar with some sort of pull stud in the thread in the collet and then make some kind of gripper that is down in side the spindle like on 40 and 50 taper machines? The problem you may run into here is that the R8 spindles are small in diameter and you may not have the space to do this.
An open mind is a virtue...so long as all the common sense has not leaked out.