Got it working (for PHP sessions) ... you have to change the file mode for the session save path, e.g.:
session.save_path = "0;0660;/path/to/sessions"
And also remove the following line from the .php-fcgi-starter file:
-d session.save_path=/path/to/tmp \
... because it overwrites anything you set in your custom php.ini settings (bug?).