The dress code is the WDW version of business casual--as printed in the "Handy Guide to all the Magic" the code is:
"Men may wear dress pants, jeans, trousers or dress shorts along with short or long sleeved shirts (collar required). Women may wear jeans, skirts, dresses or dress shorts, with a blouse or sweater."
Note that no mention is made of footwear.
It seems the that shirt code for men is "fixated" on a collar. But for women, a "collar-less" sweater will get you in. So it seems any sort of T-shirt is out for both groups.
Would the code be enforced at CG?? Who knows?