Hmmm, structural loads;
As I think about it, using an unsuspecting block of concrete at the tail, to balance your aircraft in flight, might well, on the whole, reduce structural loads.
We know that the tiedown itself is good for 700 pounds or so, so we're not worried about loading that. We know that the tail produces a balancing down force at all times the aircraft is in level flight, so if the aircraft is flying level, the loads through the tail boom must the same.
That just leaves the loads carried by the H stab spar, which normally provides all of the down force. Now, the need for an aerodynamic downforce from the H stab, is greatly reduced. So the load on that is relieved!
But to have though of that, I must have too much time on my hands tonight! Back to a bit more work!