I am familiar with the Howey case (and the Shavers case, which is where it came up). Buying a non-Zen solo hashlet is a different thing because what you're primarily buying is processing time. It's like buying time on a mainframe back in the 80s. Although there might be a small amount of maintenance, the computer is already set up, and you're renting timeslices on an item. In Howey and Shavers, the investors were primarily buying the efforts of people (harvesting, selling, etc.). Here, you're buying time with items.
Now, ZenPool is different because he's admitted that he does other activities that create the profits enjoyed by those "mining" the pool. That's where the Howey test scores big. For the other solos, I don't see it.