Currently there is no straightforward way to achieve this. To accomplish this scenario you will have to play around with editing the ControlTemplate of RadTileViewItem and RadTileView's ItemTemplate.
I've attached a sample project demonstrating how this can be achieved. We have plans to optimize this process in the future. Have a look at the process and let us know if it helps.
On a side note, I'd like to inform you that we've just released an online tool that allows you to reduce the size of your Silverlight applications. For more information, please visit http://blogs.telerik.com/blogs/posts/10-06-10/telerik_assembly_minifier.aspx
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