The main reason for reducing spoke count is to improve aerodynamics. This usually goes along with a deeper-section rim, which makes up for the strength lost with the missing spokes. Generally when there are fewer spokes, each spoke is at a higher tension, although that's not always the case. Highly tensioned spokes are more likely to be pushed past their limits of elasticity, which leads to breakage.
Too few spokes will increase the likelihood of spoke breakage. I'm not sure if 24 is "too few" in this context. The effects of a broken spoke are magnified when there are fewer spokes--the wheel will go farther out of true.
The fact that you'll be using a dyno hub will not affect the sturdiness of the wheel. I think this comes down to how handy you are, and how risk-averse you are. You should at any rate be carrying spare spokes for an ultra event. You'll be buying a new hub and new spokes anyhow; you could buy a new rim for added peace of mind.