My friend Colleen is the owner of Tutuvi. She designs both fabrics and clothes, including T-shirts.
She said that a T-shirt is flat enough to be considered two-dimensional, so we can think of it as a surface on which we print a picture.
Some people, she said, will create a screen that is almost as large as the shirt and will press it against the shirt, transferring the design in its entirety.
Others will use a machine with arms that carefully draw the design on the shirt.
She liked Michele’s design (as many of us do) and said that two parts of it would be hard to replicate: the lines on the sleeves, and the words that curve around the neck. She did feel that the design could be reproduced, though not in bulk. Each shirt would have to be printed carefully, which would take time.
Now, these are my recollections of what Colleen said. It would be best to communicate with her directly if one were going ahead with the T-shirt design.
Also, we did not talk about the heat-transfer method.
I hope this is helpful, @micheleminno