Answer:

I’ve figured this out myself.

Add the following code to your theme’s functions.php file:

// Plugin by http://wordpress.org/extend/plugins/peters-login-redirect/
// Solution by http://www.dominornovus.com

function login_widget_redirect( $empty, $redirect_to, $requested_redirect_to, $user )
{
// If the referring page contains "/forums/"...
if (stripos($_SERVER['HTTP_REFERER'],'/forums/') !== false)
{
$referringpage = $_SERVER[HTTP_REFERER];
return $referringpage;
}
//Otherwise, check the referring page contains "/blog/"...
else if (stripos($_SERVER['HTTP_REFERER'],'/blog/') !== false)
{
$referringpage = $_SERVER[HTTP_REFERER];
return $referringpage;
}
// Otherwise, default to the plugin's redirect settings...
else
{
return false;
}

}

add_filter( 'rul_before_user', 'login_widget_redirect', 10, 4 );

This will effectively disable the plugin’s redirect settings anytime a user logs in from a forum or blog page. The standard wp-login.php page will inherit the plugin’s redirect settings.

You can find a detailed explanation of the code at my blog:

http://www.dominornovus.com/2011/11/14/restrict-peters-login-redirect-to-same-page-redirection/