"Breast feeding is work ... you have to be very careful with what you do/don't consume..."
This is a myth which undermines breastfeeding. You don't have to be very careful with what you do and don't consume as poor diet affects the mother rather than the baby.
women's bodies are producing, on average, just enough and no more."
So how did wet nurses manage then?! Another misunderstanding.
Yes, a lactating woman produces exactly what is needed, but that supply can be increased and decreased as needed.
Think about it. If a wet nurse was given another baby to add to her remit, she'd put it to the breast and soon enough, her supply would increase to accommodate her new charge. Likewise, when our own babies go through growth spurts, we put them to the breast or let them feed for longer order to increase our supply.
It's very simple. If the OP's idea was a serious one - and I doubt it ever will be, but if it was - it would be easy enough to increase a woman's supply beyond what is needed for her own baby. After all, this is exactly what diary farmers do with their cows....!
I know these two points are slightly off topic, but it's really undermining to see these sorts of things passed off as truth.
The reason a lot of women don't even try, or quickly give up breastfeeding, is because they don't think their diet is up to the job. How do you think malnourished women in famine-ridden countries manage? They might not be getting enough nutrition, but their babies certainly are.