One bit of ad hoc change to the uniform, and they fall on the player like a ton of bricks. Make a political statement on the field, not so much.
NFL rules about uniforms are just as strict as the rules about the game itself. Why? Because these meticulous rules are written into licensing contracts that the NFL signs with the companies that produce the equipment the players wear on the field.
If Nike is paying $X million per year to the NFL as part of a licensing/merchandising contract, then you can be damn sure that they're going to force the NFL to write very strict rules against any "ad hoc change to the uniform" by any of the players.