Fact is, human milk retails for over $20/ounce. Any mom who would be freely willing to trust that to anything less than fully insured air cargo would have to have rocks in her head. Someone at TSA *finally* used some common sense and realized that most moms who would be transporting breast milk would NOT be travelling with the baby, and besides, if you're lactating you're transporting it anyway; it's not like you can stop production. I can leave home empty-handed and arrive at a destination 8 hours later with 26 ounces of bottled milk.
Most TSO's have never breastfed, mostly because of being male, but even the women are clueless, IME. I'm sure that it seems an entirely reasonable idea to most of them that a baby could not be breastfed on a plane (in public close quarters with strangers), though we know that's total nonsense.
Use the ignorance to your advantage. If you have a right to breastfeed on a plane you would reasonably have just as much right NOT to, and to carry any bottles baby might need, just as a formula-feeding mother might.
I fly frequently with children, and my rule is ALWAYS CARRY 24 HOURS WORTH OF *ANY* FOOD/TOILETRIES/MEDICINES/CLOTHING my child *might* need, plus a change of clothing for the adults. There is no way of knowing when you enter an airport just how long you will be there, and especially just how long you will be on a plane. I've sat on runways for hours times past counting, and I've been thrown up on as a consequence when it happened in summer. I've had children develop the runs and need twenty diapers for a 3 hour flight. I've left home for a 30 minute flight to Chicago and not walked into Midway Airport until 13 hours later. You just don't know, and TSA doesn't know either. No one knows, and 24 hours worth of food and supplies *IS* a reasonable amount for air travel, in any circumstance.