I've got a bit of a weak core/abs, and I frequently DL/Squat/Press heavy, and train various events.
Doing the events, (Specifically stones, kegs, and logs), hasn't really improved my core strength appreciably, maybe because I don't train them that often. But, I could certainly see how they could be a good training stimulus - try shouldering a heavy ass stone, and as you slowly (in my case) roll it up your body, you'll know whether or not you've got weak abs.
I have found that isometric core work has helped me, given that in lots of strongman events, you're just hoping to God that your spine will hold it's ground, and not snap in half. Some of the tougher plank variations have been good.
Oh, and the evil wheel is a keeper, though I don't use it enough myself.