Hydro, the seed will die and will rot if it lays around for long. The thing is, not all of it will die or rot and some will still be viable when the weather gets right. If you stop and think about how many seeds are actually in a lb of grass seed, and how many square inches there is in a sqft or 1000 sq ft. You will probably figure out even low seed rates will result in several seeds per square inch of area being seeded. This means quite a large amount of the seed can die, rot, or simply not germinate, and you can still have a very thick stand of grass. An example, 1000sqft is equal to 144,000 sq inches of area. one lb of fescue or rye grass seed contains over 220,000 individual seeds. Now multiply that by some of the outragous recommended seed rates and you can easily have 4 to 8 seeds per square inch of surface area. You could lose 75% of your seed to die off or rot and still have a beautiful stand of grass once the weather decides to lend a helping hand. Just one reason it doesnt pay to get in a big hurry to do a reshoot if the job seems to be coming in thin. Usually just a little more time is all that is actually needed.
Now, if that seed sprouts and dies, no amount of waiting or weather change is going to bring that lawn in. Just another reason to let the seed lay if you dont have the capabilities to properly irrigate the newly seeded grass. Nothing worse than watering the heck out of the grass to get it started growing and then deciding you cant keep up our watering schedule. all you have done is waste a bucnh of money because its not the hydroseeders fault you didnt water.
I just want to add, if you are going to be hydroseeding in hot dry weather, it is best to simply leave off any amounts of nitrogen in your hydroseeding slurry. 10lbs of urea nitrogen, per acre, in direct contact with seed can result is a 50% reduction in germination rates due to gasification and the resulting seed kill. Seed water, and fertilizer mixed in a hydroseeder, pretty much guarantees fertilizer and seed contact, and hot weather and water pretty much guarantees urea gasification.