I have successful modified modules in CF7 to do this.
I have replaced in modules/text.php:
esc_attr( $value ) => esc_attr($_GET[$name] ? $_GET[$name] : $value )
and, in textarea.php:
esc_html( $value ) => esc_html($_GET[$name] ? $_GET[$name] : $value )
Use with:
http://your.web.site/contact/?your-message=precompiled%20text
Warning: this code is potentially not secure! Use at your risk!
