Hello,
According to the knowledge i have , BING search API by default has geo-filtering supported .Quoting from BING search API FAQ ."Search expressions and their corresponding Bing Uniform Resource Identifiers (URIs) that are relevant to your query. For example: if your query is San Francisco, the Bing Search API returns URIs you can use to get search results about San Francisco Weather, San Francisco Travel, and San Francisco Things to Do. You can use the related search expressions to suggest more specific queries to your users. " .
So i am unable to understand what kind of geo filtering support you wish to add. :)