I think it's more rational to ship the hydrogen as methane, rather than ammonia. The energetics are comparable IIRC (Sabatier for methane, Haber-Bosch for ammonia, but it's more or less the same kind of high-pressure moderate-temp chemistry pipeline). Methane from CO2 + H2 vs. ammonia from N2 + H2, it's just that the latter technology has seen more research and investment.