Just to offer an answer to your original question, the only other really private space that's probably readily available is the car if you guys drive. The only time I've used this option personally is for my own comfort (e.g. at grocery store, can't nurse a newborn standing up too comfortably, literally nowhere to sit but a hard floor). But you could do it, a car is like your own space.
However I think many of us just cringe when we read statements like your husband's that speak to a mindset that we hope is old fashioned and on its way out in this culture.
You definately are doing an awesome job mothering your dd but I"m sorry you have to confront these kinds of issues and negativity around giving your daughter the food she needs.
Reading over again this part feels really sad:
[QUOTE/]What will I say to her when she asks why I leave the room to nurse her little brother or sister? I'm guessing something along the lines of "Some people are not comfortable with me not using a bottle to feed DC2 so I go to another room so that I'm not being insensitive to their feelings and am being respectful of what they feel." I don't know if that answer would make her averse to nursing her own child one day. I pray not but it's not something that I can control.[/QUOTE]
That made me feel really sad for your dd! Flip it around with words that make it clear that drinking human milk is the norm for a baby human! Rather than buying the starting assumption from your dh and his family that bottle feeding artificial milk is the norm and you are a deviant.
It does sound, though, like there are some cultural differences at work here; maybe your husband's family has been inculturated a certain way, or he was raised that way, and while I disagree I just wanted to recognize that these things can be slow to change. You can let him know you respect his views while not choosing to change your own actions based on them. This happens sometimes in marriage, it doesn't mean you don't love or respect him (or his relatives).
You have a good idea in suggesting your dh might like hard facts - there are some great resources on the "stats" on breastmilk's importance for health, brain growth, reducing asthma etc. Maybe someone else has a link?
Meanwhile realize that cultural perspectives can be slow to shift but think of yourself as being a pioneer in that little subculture - your own daughter,maybe your nieces and nephews or whomever, will learn from your example, so be proud of the gift you give them!