Dear Björ,
The add_filter runs the hook_search_custom_fields() function, so it is a kind of entry point.
You would have to additionally use this hook: http://wp-types.com/documentation/user-guides/views-filters/wpv-before-display-post/ And parse the $post->post_content to only show this section. This requires some more intermediate PHP programming.
This may help a little: http://stackoverflow.com/questions/2483844/highlight-the-word-in-the-string-if-it-contains-the-keyword
Please let me know if you are satisfied with my answer and if I can help you with any other questions you might have.
Regards,
Paweł