I have a book that we are checking out for the ABA library called The Breastfeeding Mothers Guide to "Making More Milk" is quite American as is recommended by La Leche League but this is what it says about making more milk when pregnant

"What are your options for increasing supply while pregnant? It really is a case of swimming upstream against nature, because your body shifts priority to preparing for new baby - nursing more frequently may help.
Using Herbs to Increase Supply While Pregnant
If you want to continue breastfeeding during a new pregnancy, a galactogogue may help maintain some of your milk production. However you're working against nature as your body's first priority is to prepare for the new baby. Not al herbal galactogogues are appropriate for pregnancy. Those generally considered safe for pregnancy include red raspberry, nettle, oat straw, oats, dandelion leaf, alfalfa, fennel and milk thistle"

Not sure if that is any use or not.