I have a solution! I've designed a onesie, which can obviously be a t-shirt as well, with a pocket and opening for the cable. These are very cute. I am in the process of getting the design patented so it can be manufactured in bulk. No more visits to the fabric store and sewing pockets yourselves! I have the pocket on the front or the back. You'll have to let me know which is preferred.
If you are interested in this PLEASE comment and let me know. If enough people are in need of this, I will pursue manufacturing this!