If they want to waste their money and at the same time raise funds to pay for the new stadium... whatever. The bricks can easily be removed or even defaced by the more rabid fans, so its not like some Seahawk message is going to be there forever even if they sneak something past the censors. They're doing this for attention so once they brag that it got through, it'll eventually come to the attention of the 49ers and they'll get rid of the brick.
To clarify, I'm not being critical of the Hawks' fans that want to do this. I'm being critical of Niner fans who are brushing it off as if it's no big deal that also wanted Kaepernick imprisoned for wearing a Dolphins' hat.