I used to own a 1997 BMW F650 Funduro. There was a French company that manufactured a fully enclosed chain guard for that bike and I bought one. However, I was apparently the only person that did, as they stopped marketing the product shortly after it was introduced. It really worked great for keeping my drive chain clean and well oiled. But it was a bear to deal with when adjusting the chain or removing the rear wheel to change the tire.
My feeling is that it is going to be tough to come up with a design that fully encloses the belt, as well as allowing the tension to be easily checked, the belt to be adjusted and the rear wheel to be removed without too much hassle. The other thing to consider is will enclosing the belt cause it to over-heat and thereby affect it life?
I think the best approach would be to go for a partial enclosure that includes a hugger with more coverage than stock and an attached belt guard that covers the upper and lower sections as much as possible, while leaving the outside of the belt exposed. Unfortunately, I have bought plastic hugger/chain guards similar to this design for my motorcycles in the past and they were not all that cheap. Prices ranged between $200 and $400 for the products. I really doubt that one could be produced and marketed for less than $400 in the small numbers that would be bought by Zero owners.