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:
http://wordpress.org/extend/plugins/contact-form-7-dynamic-text-extension/
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);`
where $fleet_selector is the selector I made..
On that select box,
onchange='document.getElementById('fleet').value=this.value;'
and I also added a javascript snippet to first populate that box on page load with a preselected value for the dropdown, as they arrive on my form after first picking something somewhere else, and that value is one of the dropdown values, I added select='selected' to the value in the dropdown as well..
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..
The Todd