Chapter 46, Title 10, Sec.46.035 UNLAWFUL CARRYING OF HANDGUN BY LICENSE HOLDER. (a)A license holder commits an offense if the license holder carries a handgun on or about the license holder's person under the authority of Subchapter H, Chapter 411, Government Code, and intentionally fails to conceal the handgun.
(3) "Concealed handgun" means a handgun, the presence of
which is not openly discernible to the ordinary observation of a reasonable
These laws would seem to state that using any means to make a person aware of the fact that you have a firearm would violate the law.
EDIT: Sorry, these are Texas laws. Not sure about other states.
The tshirts would be awesome to wear anyhow, even if CC was not required... They are kinda cool, sort of like OC without actually seeing the firearm. Would definitely make someone think twice.