Is it possible to retrieve partial forms?
This is how I might end up doing it:


When a JavaScript callback is triggered by a user editing the body field (this can be done via the #ajax property), clone current node found on form_state.
Create a render array for the whole node by using FAPI's drupal_build_form($form_id, $form_state).
Extract the body field from the render array.
Render the body field.
Return HTML to the front-end (JavaScript) so it can be placed on a preview DIV.