Yes indeed, those were the good old days when the scupper cart had been invented but the upper hole had not yet been discovered!
Hobie carts then had 7" legs and I used mine (still have it) in the "incomplete" scuppers. Obviously the boat was designed to be used this way or it would not have had the scupper holes in the first place. As I recall, the lid is pretty thick and can take some weight. If I had this boat today, I (personally) would get a basic cart, fit it in the boat upside down, adjust the cart clamps to achieve a double seat (clamp touching bottom of hull and leg toughing scupper lid). Then:
1. measure the gap between bottom of hull and cart crossbar
2. measure the distance between top of cart clamp and top of cart leg
3. take the lesser of the two, add 1/2" for safety, and cut that much off from the cart legs.
In the end, I would have the boat sitting low on the cart as designed, the cart seating on the lid of the scupper AND at the bottom of the hole without touching the crossbar. Since Hobie doesn't recommend using the scupper cart on this model now, I would not recommend it either, especially if the scupper tubes appear to be thin as noted with a flashlight test.