I would dilute the glue in a bowl, 1:1 with water, and use a small roller painting brush to roll the glue on the batting. Someone recently posted that this was very easy and fast!
I would not try diluting and spraying the glue on. Aside from the problem of the glue gumming up the sprayer, the repeated squeezing of the trigger to dispense the glue is very hard on the hands.