Node tabs leading to related nodes
I used panels for this. I added a new panel, which has the node with the nid of the webform as only content item in the main content area. I set the menu entry for the panel to "Menu Tab" and set the access rule of the panel to "Show only when node is of type person". This shows the desired tab element when a person node is selected.

For the webform itself I used the hook_form_alterwith the $form_id == "webform_client_form_&lt;nid&gt;" having &lt;nid&gt; as the node ID of the webform to set the #default_value of the webform field with the data coming from the person node. The ID of the person node I get by using the arg(1) function and the loading the node data with node_load($nid).

Hope that helps others.