You may have seen this video before but it sounds to me like you're doing something very similar to what these farmers are doing in the video. Basically, you�re just transitioning from a cover crop to a cash crop. You can drill into green standing wheat or rye. I think I�d just let the plot grow up....drill into it, then spray immediately after drilling...terminating last falls plot. The cereal grains in your mix will use up a lot of water when they start going to seed. Its best to terminate them prior to that point and let your new seedlings have that moisture.
http://www.youtube.com/watch?v=nWXCLVCJWTU
