Sorry, what?
Empty bottles are restricted?
Of course they're not - you can carry as many empty bottles as you'd like in your hand luggage if they fit! - we always take empty water bottles for instance, so that's 4 litres of empty bottle that's been through multiple airports across the world, and no-one has batted an eyelid, and why on earth would they?
If you want to carry large amounts of breast milk about, you'll have to check it. If you're travelling on a budget airline, that costs extra. That costs whether you're carrying breastmilk or nappies, or you're 6'6" and need more space for your larger clothes.
I think that in this case, what someone's wanting to do something quite so unusual as carry litres of liquid around, they are the ones that should bear the brunt of that cost.
Mind you, given how luggage handlers chuck suitcases about, I don't know that I really like the idea of litres of milk in a suitcase either.. I had a vodka bottle smash once and that caused enough havoc.
I completely support breastfeeding and obviously I'm a feminist, but I can't see that if you have a young baby, you can carry on exactly as you were - something has to give - either you'll need to travel less, or use formula a bit when you haven't been able to express enough. I don't think that carrying 3 gallons of breastmilk through heathrow is really a practical or s