It's almost a certainty that this will be cheaper than a Y fare, although it will have considerably more restrictions. You can search for a Y fare by using Advanced Search and entering the Y fare code.

I'm not sure your interpretation of the travel policy is going to mesh with that of whomever is approving your expenses, though. You're basically comparing the cheapest first class fare to the most expensive coach fare.

To answer the question you actually asked, unless you're flying on a p.s. route (EWR-LAX/SFO, BOS-SFO, or vice versa), the Z fare is likely a differentially-priced fare with a corresponding underlying economy fare. (The first letter of the fare basis is unlikely to be 'Z'). However, UA has changed the way that they code those fares so that they'll look more like 'true' first class fares now.