My goal has always been to simply map the Harmony Hub's features as standard Hubitat Capabilities, nothing more. The problem is that there are a vast number of devices supported by Harmony, each with unique lists of features/commands available. This makes it very challenging to try and find a single solution that fits well for Harmony 'Devices'. Thus, the focus has been on the Harmony 'Activities' which are much simpler. I personally cannot understand why anyone would want to use a smart phone or tablet to interact manually with a Harmony Hub. It just is not an intuitive interface. They can simply use the Harmony app if they are so inclined. I personally use the physical Harmony Remote control for 99% of my interaction with my home theater system. I do have Hubitat start and stop Activities based on things like "Good Morning" and "Good Night" routines that I have implemented.
Not sure how these would be exposed via MakerAPI, as detailed JSON data does not fit into the Hubitat standard capabilities model. Perhaps your application might be better off simply connecting directly to the Harmony Hub on the local network? There are plenty of Open Source code examples that you could use in Android to directly connect to a Harmony Hub. Just a thought...