You can easily define routes in custom fields. It’s necessary to get the field URL with something like $this->model, 'field/' . $this->name because the routes are relative. Thanks to @lukasbestle for mentioning this in this thread: Routing in custom form field.
But if I put that field into a structure or builder field, the route is not correct anymore.
It will still output the same relative URL, for example http://domain.com/panel/pages/home/field/text/simplemde/.
Is it even possible to use a field router inside of another field?