Heavy Pallof press holds. Using a, duh, heavy weight and holding for a count of 5-10 per side, usually 3-4 sets. Great static ab/"core" drill that trains anti-rotation and is some nice "indirect" oblique work.
Use nice wide, stable base, set the cable around or just below shoulder-height, and keep the weight close to you as you step back from the stack into position. Start counting as soon as your arms are extended - Imagine holding the peak of a cable crossover, arms are rounded bear hug-style or a bit more straight, but definitely not locked out.
I also like holding back extensions at the peak for time instead of doing reps. I could've sworn I read Wendler first mention the idea a long time ago, but I can't find it. Theory being that the low back needs a bunch of static strength for maximum health and performance, so build that muscular endurance.
Basically get into position for hypers, do half a rep, and then hold the top position (straight line from heels to neck) for a 15-30 count. Hold weight as needed. Doesn't work as well on the 45-degree back extension setup. The standard, with body parallel to the ground, is obviously better suited.