Hello Rac,
You cannot get the rendered output of the RadChart control, nor the ChartImage.axd url like this. You can still use the RadChart.Save(...) method API to save the image file on the server and then serve it via your own custom HttpHandler to the client-side e.g. from the page method you will need to return your custom HttpHandler url that points to the saved image on the server.
Regards,
Manuel
the Telerik team