If I were to Implement the system, this what I would do:
1. User in USA: Instead of sending SMS to Kenya number, I would prefer the user just to log into RapidSMS System and create outgoing message and probably assign the country the SMS should be sent to.
2. In west Africa. I will have android phone server installed SMSsync or Telerivet. This will check the Kenya server for any new SMS that has not been sent and sent out. Users in West africa will Reply to the Number attached to the Android phone. SMSsync/Telerivet inturn push the incoming SMS to Kenya Server as reply to the original SMS.User in USA can check response on RapidSMS interface.
According to me this will keep cost down since SMS will be charged at per country local rates.
Am open to criticism and further suggestion.