I only have handheld Harmony remotes, so I haven't ever looked at integrating the hub.
But this library looks interesting in general as a template for local control that could be ported to Hubitat.
Though this (brief) discussion makes it sound like button presses aren't able to be handled locally, which is what I think you are looking for: Can this be used in the other way, to monitor for commands? · Issue #33 · ehendrix23/aioharmony · GitHub