Wowza cannot meet this exact requirement at present: vod pre-roll before live across client types.
You can do vod before live with Flash RTMP clients only, but it is not all serve-side. You need to listen for "NetStream.Play.Stop" notification in NetStream onStatus handler when playing the pre-roll then play the live stream.
You can do intermittent insertion, like tv advertisement using the Stream class or the IPublishingProvider API. Here are some examples: