I kept two hoplos for a while in a dirt substrate tank. Big mistake, as they dig a lot (I suspect they were trying to spawn). The tank was an absolute disaster until I could catch them. Other than that, though, they are a wonderful fish, and in a tank with plain sand they would be fine. You'd need to make sure that any heavy decor is grounded on the tank bottom rather than on the sand; otherwise they might dig under it and crush themselves.
I agree that a 280L tank would be too small for them to really live well. Mine were very active and seemed to enjoy exploring. In fact, you kind of have me wanting to try them again...