There was a program started in Canada to fit a 737-200 with a belly mounted retardent tank for aerial forest fire suppression but I would be very surprised if a spray rig would work. Spray systems consist of a series of nozzles below and behind the wing. The flow rate is fairly low and a lot of work is put into palcing the nozzles so that they exhuast into smooth air. The underwing mounted engines of a 737 would put so much heat and turbulance into the pattern I would guess you could only put nozzles at mid span and outboard which would put a big gap into the middle of the pattern. Also I cannot think how you would mount the nozzles and still allow full flap operation.