So far Aspect_Window and graphic driver in OCCT do not provide an API for rendering into window sub-region (save the Graphic3d_Camera::SetTile() property which has another purpose). Though this is technically possible, this will be an advanced usage scenario, as application will be responsible for clearing/filling outer parts of the window and for swapping front/back buffers. So you may propose a patch for OCCT adding such a feature.
Using existing API, this could be done by specifying an off-screen render target (Graphic3d_CView::SetFBO()) instead of a window and manual blitting of FBO content into window buffer.