I can imagine essentially a grid with a moat of computationally very difficult structures to send commands through without help from inside the moat. I can see how you'd, like, send a password hash through and something would come out and "get you" for lack of a better explanation.
But that seems like it would be about as secure as a real moat.... Which is to say, it's still only as safe as your expectations about your opponent's capabilities are accurate. So, you might have some ideas about how malware must do computation, and you can have your little computation immune system working to make that stochastically impossible. But then the malware writers will figure out weirder ways to do computation. Like maybe they give up on penetrating the moat and have the immune system do the nefarious computation. Or whatever. My point is that you still end up in an arms race, except it's an arms race within a complex system so it's even more difficult to have an understanding of your opponent's capabilities than it is inside a von neumann machine.