Give weighted leg raises a go (in that weird 'chair' kinda thing, can't recall its proper name). Sure it fatigues the quads and works the psoas a little more than abs, but when you've got an 8kg or more dumbbell hanging from your feet without bending the knees, it does a good job of everything my very basic ab circuit is these weighted leg raises, to that stupid ab crunch machine, to standing oblique crunches (20kg in ONE hand then swap), to 90 second plank. Repeat.
Also how you liking the wood chops Birds? I've been doing them with the cable up a bit higher, then crunching forward more. Takes the back out of the equation a bit and focusses more on obliques.