The reason this doesn't currently work is that I wasn't sure how to process message codes for preview messages. The problem lies in the fact that we can't determine the list number, or there may not BE a list number. Therefore, we cannot determine which list-specific codes to process and, if codes were processed, the preview might not be an accurate indication of how your message(s) will appear. I will move this to the suggestions forum so that I might work on it for a future release. Sorry about that...
Regards