This took a while to find and could use some work yet, but try this in your functions.php
file:
add_action("wpcf7_posted_data", "wpcf7_modify_this");
function wpcf7_modify_this($posted_data) {
// if user leaves the message area blank, set to "None"
if ($posted_data['my-message'] == "")
$posted_data['my-message'] = "None";
// generic example
if ( /* some logic */ )
$posted_data['/* cf7-field-name */'] = "ValueToEmail";
return $posted_data;
}
One thing to note here is that validation occurs prior to this point, so if you're allowing users to enter a blank value, don't make it required.