There are non-middle seats but they are all in the back. You can just go through the booking process (don't actually pay) to see them.
On the afore-mentioned HNL flight, rows 28-32 (window, middle, and aisle) are marked as saver.
They premium seat selection is $119 from X, and $99 from main, and the difference between saver and main is $20. So it's the same price as getting main and picking as premium seat... but, you are still bound by all the saver restrictions (it's still an X fare, despite being the same price as booking G and picking the same seat). So, not a good idea.