I am not quite sure whether I completely understood your setup. As far as I understood you want to extract an ID of a control, placed in a user control in an external page, loaded in a RadPane and get this ID in the main page which contains the whole splitter control.
If I am right, I suggest to examine this KB article
which explains how to reference content page in a splitter pane from the parent page and vice versa. I suggest to implement a function in the content page which gets the desired ID by using code blocks <%%> and then call it in the main page when needed.
In case I did not understand you correctly, please provide a more detailed explanation. It will be best if you prepare a sample, demo project, open a new support ticket and send it to me. It is not needed to make it as complicated as your original one, it is enough just to replicate the problem.
the Telerik team