unfortunately this is not possible at the moment, since tags are applied on the library level, not the document level. We are planning to improve this in Sitefinity 4.0 our next major release, which will feature the new advanced Generic Content API with new implementations for categories and tags (the new API will be backward compatible).
I am sorry that I was not able to help you more at this moment. Please let us know if there is anything else we can do for you.
the Telerik team
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