Field type for selecting a section (channel / structure / single)? I have a channel for products but in some cases I would like to associate a product with a structure containing documentation and release note pages:

http://mysite.com/product/my-product/docs/getting-started
                          ^--------^      ^-------------^
                              |                  |
                           channel             topic (from 'myProductStructure')


Is there an existing field type (or plugin) which would allow selection of the associated structure type:

Title: My Product  [text]
Doc Source: My Product Structure  [drop-down]


Such that I could use that in templates?

{% for entry in craft.entries.section( product.docSource ).slug( craft.request.lastSegment ) %}
    ...
{% endfor %}
You can do this with LJ DynamicFields. In the plugin docs is an example of how to create a dropdown that is a list of all sections.