Exclude T-shirt pattern from Keyer

Hi, we shot a green screen sequence and everything looks perfect, except we realized there are tiny green elements in the complex pattern on the subject's T-shirt.

Is there a way to draw a mask so that the body of the T-shirt (which stays in the same place on-screen) is not affected by the Keyer?

I couldn't see a mask option in the Keyer, and the Draw Mask option just cuts out the T-shirt completely.

We just want to have the Keyer "ignore" a masked off area.