I'm not sure an embroidery machine can handle going through the ink used in the screenprint on many T-shirts. The ink is layed down very think on some T's and many people have reported problems with their machine being able to go through it without problems. Many quilt around the imprints.
Definitely do a test but be prepared for skipped stitches. Additionally, T-Shirt quilts are very heavy. You will need to devise a support system for hooping unless you plan to do this in a quilt as you go style.