Hmmm I think I would create a custom Work Item type for storyboards, and maybe link to them the document with the complete storyboard (keeping it on Sharepoint), and use the work item to assing it to the iterations of the TFS project.
You can give access to this to our customers for priorizing them, if they are external users, a web based interface, like Team Plain, or even create your own, customized, and as simple as you need, web based tool to make changes to this work items.
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