[ZTAngel]

I think that having them rush out of ATO is the most fair option. Neutral would be if they had a house of their own like you.
I go to UCF where four of the ten sororities are unhoused. The four unhoused sororities rush out of the fraternity houses. The PNM's do not associate the sororities with the houses that they are rushing out of. Girls will not join Delta Zeta for their supposed ties with ATO; I am sure DZ will make it clear as possible that it is not their house....they have to.
You have more of an upper hand in this situation than you think. From my experience at UCF, I have had a number of girls that are going through rush tell me that they would not join a sorority that does not have a house. Why? I don't know but for some reason this is what the PNM's prefer. It hurts a sorority more to not have a house and to rush out of fraternity house more than you think. I think that instead of being "upset" you should try to be supportive. It must be hard for a chapter to go through formal rush for the first time and not have their own house to rush out of. Y'all have had the same house for years and therefore know the structure which means more organized rush parties and entrances/exits. Delta Zeta not only has to learn how to rush these girls but they have to organize their parties in a house that is completely unfamiliar to them. They are a new sorority and need as much support as possible.
When I went through rush, the unhoused sororities made it clear that they were rushing out of house that wasn't their own. Our Rho Gamma's also informed us of this. The PNM's will know that the ATO house is not DZ's.