Is there anything in place (like a popup after login) to draw attention to new posts in that section, though? Because I'm not sure I would notice it myself (at least not quickly).
If it's time-sensitive (i.e. the more people that notice it quickly, the better) then maybe it's a good idea to temporarily have a stickied post on the parent board (Bitcoin Discussion) that refers to the one in "Important Announcements"?
I believe one of this is necessary though, because i don't think that board is popular, especially among newbies that the message may be most important too, newbies are more active in the beginners and help board than bitcoin discussion, and even in bitcoin discussion i don't think many of them visit the child boards of which important announcement is one of them.
If that thread is made in important announcement board, i'm sorry but i think mostly those who already know they should not hold BTC in online or centralized accounts are the people that will see it very early, and newbies and other junior members may only get to know about it if their attention is drawn there in someway.
I suggest that if the thread is made, there can be a voluntary campaign for people to include the thread link in their personal text, like many of us did with NotATether's bitcoin cleanup campaign, it will get more attention that way and we don't have to disturb Theymos for a 'popup after login' or to have it stickied