I just did this for a client.
Just create your own shortcode (functions.php)
function cf7_add_sku(){
global $product;
return $product->get_sku();
}
add_shortcode('CF7_ADD_SKU', 'cf7_add_SKU');
Then add the dynamic field in your form
<p>Subject<br />
[dynamictext your-subject "CF7_ADD_SKU"]</p>