我还遇到了在XML中进行令牌响应的需要.
我提出的最佳解决方案是在Accept头中声明时实现将JSON转换为XML的HttpModule.

public class OAuthTokenXmlResponseHttpModule : IHttpModule
{
    private static readonly string FilterKey = typeof(OAuthTokenXmlResponseHttpModule).Name + typeof(MemoryStreamFilter).Name;

    public void Init(HttpApplication application)
    {
        application.BeginRequest += ApplicationOnBeginRequest;
        application.EndRequest += ApplicationOnEndRequest;
    }

    private static void ApplicationOnBeginRequest(object sender,EventArgs eventArgs)
    {
        var application = (HttpApplication)sender;

        if (ShouldConvertToXml(application.Context.Request) == false) return;

        var filter = new MemoryStreamFilter(application.Response.Filter);
        application.Response.Filter = filter;
        application.Context.Items[FilterKey] = filter;
    }

    private static bool ShouldConvertToXml(HttpRequest request)
    {
        var isTokenPath = string.Equals("/token",request.Path,StringComparison.InvariantCultureIgnoreCase);
        var header = request.Headers["Accept"];

        return isTokenPath && (header == "text/xml" || header == "application/xml");
    }

    private static void ApplicationOnEndRequest(object sender,EventArgs eventArgs)
    {
        var context = ((HttpApplication) sender).Context;

        var filter = context.Items[FilterKey] as MemoryStreamFilter;
        if (filter == null) return;

        var jsonResponse = filter.ToString();
        var xDocument = JsonConvert.DeserializeXNode(jsonResponse,"oauth");
        var xmlResponse = xDocument.ToString(SaveOptions.DisableFormatting);

        WriteResponse(context.Response,xmlResponse);
    }

    private static void WriteResponse(HttpResponse response,string xmlResponse)
    {
        response.Clear();
        response.ContentType = "application/xml;charset=UTF-8";
        response.Write(xmlResponse);
    }

    public void Dispose()
    {
    }
}

public class MemoryStreamFilter : Stream
{
    private readonly Stream _stream;
    private readonly MemoryStream _memoryStream = new MemoryStream();

    public MemoryStreamFilter(Stream stream)
    {
        _stream = stream;
    }

    public override void Flush()
    {
        _stream.Flush();
    }

    public override int Read(byte[] buffer,int offset,int count)
    {
        return _stream.Read(buffer,offset,count);
    }

    public override void Write(byte[] buffer,int count)
    {
        _memoryStream.Write(buffer,count);
        _stream.Write(buffer,count);
    }

    public override string ToString()
    {
        return Encoding.UTF8.GetString(_memoryStream.ToArray());
    }

    #region Rest of the overrides
    public override bool CanRead
    {
        get { throw new NotImplementedException(); }
    }

    public override bool CanSeek
    {
        get { throw new NotImplementedException(); }
    }

    public override bool CanWrite
    {
        get { throw new NotImplementedException(); }
    }

    public override long Seek(long offset,SeekOrigin origin)
    {
        throw new NotImplementedException();
    }

    public override void SetLength(long value)
    {
        throw new NotImplementedException();
    }

    public override long Length
    {
        get { throw new NotImplementedException(); }
    }

    public override long Position
    {
        get
        {
            throw new NotImplementedException();
        }
        set
        {
            throw new NotImplementedException();
        }
    }
    #endregion
}