As a former incarcerated person: no... and hell no... no one would want this.... a "personal" message played on a radio station? No... we -inmates- want to TALK with our loved ones, not plaster a letter to a bulletin board for everyone to read. 0 inmates would do that... they would rather write a loved one a letter.
Why would we anyone want everyone with a radio hear what im saying to a loved one? And how would I know when this message would be played? No, this is a terrible idea. Even if this whole process was free, it would be useless.
Im sorry, I dont mean to be such a downer, and I appreciate your desire to help the incarcerated, but I dont think this idea has any promise.