Having stepped through the code with the debugger it really doesn't look like there's a way to configure Zen to behave that way.
So, in /zen-cart/index.php ('zen-cart' = your zen-cart install folder), right after:
/**
* Load common library stuff
*/
put this code fragment:
if(isset($_GET['main_page']) && 'product_info' == $_GET['main_page'] && !isset($_GET['action']) )
{
$sPath = "http://".$_SERVER['HTTP_HOST'].'/WHERE_I_REALLY_WANT_TO_GO.html';
header("Location: $sPath");
exit();
}
and change '/WHERE_I_REALLY_WANT_TO_GO.html' to where you really want to go.
The idea is to redirect it when 'main_page' is set to 'product_info' but there's no 'action'. That is the state when I click my 'back to shopping' button, e.g.:
http: //localhost/zen-cart/index.php?main_page=product_info&cPath=3_14&products_id=15
where you'll see 'main_page=product_info' but there's no 'action='. You may need to tweak to taste.