hello! sorry this is so long... i just started typing and typing and typing....
our college has what sounds like the same application process for on campus housing as yours, and none of the sororities on campus have houses, we have suites. there's never been much of a problem (well that didn't get addressed and dealt with properly) between the sororities in regards to having unofficial houses because freshman and transfer women are not allowed in these houses (or apartments) where over 1/2 of the people living there are in a sorority at any time when there is a party, sorority event of any kind or alcohol being consumed for the entire first semester (we have deferred rush). these houses are labled as "pink" houses and have a sign that states as much that has to be posted in the window during parties (which does kinda suck cause it draws more attention to the house).
we also have a (i forget the name or exact proportion) proportional rule, that says that freshman women pretty much can't be involved in any activities where they are outnumbered by sorority women because that can be construed as a "dirty rush" event or a sorority event (such as one that these women you mention in your post might host for their philanthropy, for freshman to participate it would probably have to a) be held at a different location, and/or b)have either a majority of non-greek participants and different greeks and not break the proportion rule). therefore, this on campus house would be subject to these propotion rule restrictions, despite the fact that it is not actually a 'sorority house', and would therefore have limited interactions with freshmen women before recruitment. also, the sorority shouldn't be able to be mentioned during an event the house holds in the name of it's cause/program/etc., because after all it isn't a sorority house but a whatever the cause/program/etc. is house. but then this should be addressed as well by the school or residence life, b/c they wouldn't be allowed to host such events as anything other than the group that applied for the housing, does that make sense? (this is just how our school works, you may want to check about res. life's rules about advertising who/what is sponsoring such events in the name of that house)
however, i can understand that if one sorority has, say, many off campus houses, while the others do not, how it can be to your disadvantage. (while we have 2 adpi and a couple panhel houses, some have at least twice as many)
pnms who are very socially conscious will probably notice upperclassmen throwing around house names and associated sororities names while they are at it. ...on our campus we have some interesting house names that easily raise curiosity ... i'll be living at the castle, and then there's the barn, the ho-tel, the bakery, the chapel, the g-spot, the blueberry, i could go on....
i would suggest having your panhellenic delegate bring up this issue right off the bat when you start back. your greek advisor, vp recruitment administration and vp recruitment programming should be aware of your concerns and willing to address and resolve them. that is, after all, their job! good luck!