Thank you for the explanations, I now understand the situation. I am sorry to say, however, that RadContextMenu does not have a feature that will contain it in a certain area of the page. If you want to see it implemented in the control, you can add it to our feedback portal: https://feedback.telerik.com/project/108/Feedback/List/Feature%20Request. Providing detailed explanations on how you expect the feature to work and how it should be exposed for configuration will help our customers evaluate, comment and vote on the idea which will, in turn, determine its priority.
Nevertheless, I am attaching here an example function override that can help you position the context menu as desired. You can use that as base for further development, if needed. I must note that this is not extensively tested and I cannot guarantee it will fit all scenarios and needs.
On a side note, would you mind if I moved this thread to the public forums so other people with a similar scenario can benefit from this example?