I hopped along to the transmogging vendor guy and mogged some heirloom shoulders which I had in my bank and changed them into my tier six shoulders, then I sent these to a new level one character but unfortunately the transmog enchant was wiped in the mail.
So to quickly sum up, heirlooms can be transmogged, but the look will be lost when sent to the character. You will just have to wait to pick up a random drop or quest reward before you can make your low level look slightly different.
