first of all , this stage is moisture sensitive crop. If the plant escape this stage and survive than the plant motality will be low and plant population will bw maintained.Otherwise screening at flowering is desirable to get good results as this is most sensitive stage.

I agree. Plants are very sensitive in germination and sprouting phase. On the other hand, controlled exposure of maize seedlings to limited water supply or induced dessication could give information about their water management (especially if lines are tested), and possibility to survive stress, even in later phenophases (tolerance testing).

how about using PEG 6000 as the simulation in nutrient solution culture?

That's the best solution for physiologically induced drought, environment is absolutely controlled (growing in germination chamber or something similar) and you can achieve accurate and reproductive results. If you want to have environment closer to field, then soil media is proper, but accuracy of results will be poorer and harder to explain.