You can have an intro prompt, and position in the queue notifications, and on-hold "music". Those are your only options. However, it may be possible to have a service barge into the call and play further messages, but this would require some c# programming.
Ok, I can solve this by mixing the moh and the announcement into the moh audio file. Though it takes more more, I actually like this better because I can keep the audio track running and just soften the volume to 20% so it's more seamless.