As nobody answered here for so long time would mean that nobody has an answer. Even if you bump a ticket for the next couple of monthes, you won't increase chances to get a solution. With more luck you could open a ticket with Directadmin developers or email them directly. So please do it.
Directadmin as far as I know does not allow to customize headers which it sends to user's browser. Neither it passes headers from PHP scripts or addons/plugins. Thus there is hardly a solution as of yet which you could use with Directadmin.