Concrete weighs about 150 pcf and water weighs 62.4 pcf. I read one source that said the water ballast of a 260 is 1,000 pounds. It was difficult to find exact information, either in weight of water or gallons of water ballast. Sailboat data indicated 2,000 pounds ballast, but another source indicated that the centerboard contributes 1,000 pounds. Anyway, 1,000 pounds is 16 cubic feet or 120 gallons. That sounds reasonable. The weight of cured concrete is only 87.6 pcf more than the weight of the water in the ballast tank, so the total additional weight of ballast would be 1,400 pounds. That doesn't sound too radical when it is evenly distributed at the bottom of the hull. What's the floor area of the ballast tank? If it is just 0.5' deep on average the floor area might be 32 sf or 4' by 8'. Does that sound right? The added weight might be about 44 psf (75 psf vs 31 psf). The difference is about the live load for the floor of a residence (people and furniture). That could be significant for the stress of the hull - I'm afraid I don't know about that intuitively. Of course, if the water ballast is really 2,000 pound (240 gallons - really?) all my calculations would be doubled and I would begin to see why it would be a totally bad idea.
It doesn't sound that crazy to me. The heat of hydration in concrete can be significant if the mass is greater than 36" thick, but this wouldn't be the case. The damage from this heat is typically described for the concrete, not the formwork. If the concrete cracks, it really doesn't matter in this case. Unless it was poured on a really hot day, I doubt the heat would be significant. If the concrete was poured and the boat was placed in the water while the concrete cures for a day, there would be no problem. I can't see why there would be any damaging chemical reaction between the fiberglass ballast tank and concrete.
The additional weight on the trailer would be 2,400 pounds (total weight of the concrete, assuming water is normally removed during towing). That would be a significant weight increase, but I would guess that the trailer could be modified to handle it.