I agree it is arbitrary and your choice is a good one. I assume you changed the cg limits accordingly since Van's stated limits are measured from his datum?
For my purposes his datum is convenient because he states the distance back to the rear baggage shelf on the -8. That makes it easy to get the distance from datum to rear battery which I am removing. If someone has the distance from the leading edge of the wing to the firewall on the -8 I can figure the rest out since Vans says the leading edge is 70" back from his defined datum.
The one advantage I can see to Van's datum is all the moments have the same sign - all are CW or all are CCW depending on which side of the plane you're looking at. That eliminates a possible source of error or confusion.