I can offer a better solution, or at least one that does not require hacking the contact form 7 plugin..
I'm not sure if this is even the best way, but it works for me!
I added the plugin:
and then in the form where I want the select box to be, I add some shortcode..
[autoselectfleet][dynamichidden fleet id:fleet ]
The page is using a template,
`$content = do_shortcode( $post->post_content );
$new_content = str_replace('[autoselectfleet]', $fleet_selector, $content);`
$fleet_selector is the selector I made..
On that select box,
all this simply allowed the email to grab those dynamic hidden values and use them as needed..
Hope to have helped, lemme know if you need better explanation..