ROT13 has been implemented in v0.95.
Released today.
There is no way to detect if a message is in ROT13, so pressing the ROT13 button will always Do/Undo the entire body text (if encoded it will be decoded, otherwise the text will be encoded).
If you reply to a message that is ROT13 encoded, you first need to decode it, enter your text and then encode it again before posting to usenet.
Questions/ideas or improvements are always welcome.