I have created this timeline control out of a combination of different controls. But I get asked for this same kind of setup from different companies for different reason. I have created this control using your TreeListView to show the headers in the hierarcy while for Layer Num items, there is a list to the right which holds a list of MediaItem objects. It works but not very efficient and is missing much key functionality. And I thought because you already have controls that are similar, it wouldn't be a huge reach for you to make on like this.
So there could be two modes for this. The editing mode where the user is dragging/dropping media items on each layer item, rearranging, setting the length of media, etc. They could then hit PLAY like a Movie Maker and see a preview on a different control of what is being played. Once this is saved, it could be used in another way.
So in the readonly mode, all the items would be feed in as a hierarchtial collection. It would decribe how many levels there are and the names and order of the items. And then it would also have all the MediaItems related to each Layer Num. And then it would constantly be fed what the current time is related to this batch of items playing. The user doesn't hit PLAY in this mode but rather just sees a read only version of what is being played and watch the line goes across.
I think this would be a very welcomed control to your lineup. Because anyone who wants to make a media editing program, audio editing program, and even showing a schedule for items that happens every 30 seconds in different locations while getting a constant status of the current time would find this invaluable.