Fill the tray to the top with water. The seedling plants in their trays slowly absorb the moisture as and when they need it.
Don't your seedlings get over wet??
If I was going this route I think I would use a piece of capillary matting on the trays rather than water.
Whats in my mind is; if the weather turns really cold, then your seedlings might 'catch a cold' so to speak.
Similarly if it gets very warm the humidity around the plants might be quite high and introduce fungal diseases.
Using this process with mature plants might work but I would have thought it a bit risky with seedlings.
It is a known fact that more plants are lost due to OVER watering rather than under watering.
I hope you don't mind my comments but I would hate it if you lost your seedlings and I said nothing!