Here is a function that seems to work:

public static string UrlDoubleEncode(string text)
{
    if (text == null)
        return null;

    StringBuilder sb = new StringBuilder();
    foreach (int i in text)
    {
        sb.Append('&');
        sb.Append('#');
        sb.Append(i);
        sb.Append(';');
    }
    return HttpUtility.UrlEncode(sb.ToString());
}