No, this is not possible at the moment. It is possible to add HTML (be it a link or something else) as the value of a Custom Metadata Key-Value-pair, but for security reasons that value will be rendered as text and not as HTML in the UI. When you say that you don't want to expose the actual URL, do you mean that you want to give the link a text other than the url, like the option you have with normal HTML anchor tags e.g. link text?
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