NAPA brand silicone spray on the spring and spring pockets would be worth a try. I recommend that brand because it's easy to find and is not petroleum based so therefore won't leave a residue for dirt to stick to nor stain clothing if you rub against it.
Another idea would be to make washers out of plastic from a milk jug for the spring pockets and washers to cancel out metal to metal contact at the spring mounting points. The thin plastic would be easier to conceal.
Yeah it's the nature of the spring seats. Trying to quiet one on my Mothers bike now. Hers appeared to be squeaking at the pivot point up front not the springs. I put rubber washers where the frame attaches to pan at the front of the seat.