A good trick but I believe when you are flying a PRNAV procedure you are not allowed to create waypoints - you must use existing ones.
I stand to be corrected on this one of course!
B-HKD; you are correct in saying that fly-over points are hard coded in the FMC. What I am trying to say is that on the 73 they will change from a fly-over point to a fly-by point when you are using the direct-to function. On a PRNAV departure you are still obliged to fly-over and that is why HDG SEL is used.