Joe Tomei here. This Asahi Shimbun article about Kumamoto City FM which stayed on the air to help provide information is certainly worth a look. The webpage for the Kumamoto City FM is http://fm791.jp/
The article notes that Kosa began similar broadcasting on the quake disaster on April 23, Mifune on April 25 and Mashiki on April 27. If anyone has more information about how to listen to these stations, feel free to share them in the comments.