I would advise against it. What your insects eat typically becomes what your dragon eats. When I said you can feed the feeders spinach, I meant as in it's something ok to throw in their every once in awhile. I wouldn't use it as the bulk of their diet.
It's the reason why we can feed hornworms to our dragons only if they are fed a special diet. If you get a hornworm (tomato worm) from outside in your garden, it will be poisonous to your dragon as the tomato plant the hornworm eats is poisonous to dragons.
The healthier you feed your feeders, the better for your dragon
-Brandon