The thing is, based on a baby's requirements of around 100ml/kg/day 1L of milk is probably an overestimate for most babies whose only source of nutrition is milk. Bigger babies are usually on some solids and will be ok on less, irrespective e of whether they'd prefer more.
If you're campaigning that, say, 3L needs to be transported, it'd be pretty hard to keep it frozen solid for the duration of a flight and most women wouldn't run the risk of refreezing. So, a 7kg 5 month old baby, for example, needs to get through 4 days' worth (or so) of expressed milk to make it worthwhile, but, from memory, defrosted milk needs to be consumed pretty quickly.
Meanwhile, the lactating mother is not actually breastfeeding her baby - which is best for maintaining supply - while the baby guzzles the litres of milk it was apparently essential to take on the plane.
Have you contacted your local breastfeeding authority to ascertain whether they think it would be safe to refreeze the milk that's gone through the whole airport palaver?