Okay, I'm not Greek, but I love coming up with T-shirt slogans for anything.....why not go with a "popular brand name" theme? For example:
-"Chi Omega" in white script on a red shirt, like the Coca-Cola logo (isn't red a Chi-O colour anyway?)
-"Kappa Delta" in yellow on a navy blue background, like the Kraft Dinner logo.
-A "Trident" logo, but instead of "Trident," it'd say "Tri-Delt." Green would work best, because the green mint flavour of Trident gum is probably the most popular, but I'm not sure what the colour for Tri-Delta is, so whatever it is, that colour would probably be the one to go with.
-The "Sex and the City" logo, but instead, it'd say "Sigma Kappa and the City." (Could also work with "Sigma Delta Tau," or "Sigma Sigma Sigma.")
-Or, if you don't want to do the brand name thing, you could put your letters on the front of the shirt, and on the back, write "Inside this shirt is ONE WONDEFUL WOMAN!!! ABC Sorority."
Oh, by the way.......the A Phi/DC logo idea is SO CUTE.......and I hate heavy metal music, lol. But still, major props to whoever came up with that one.