I've seen quite a few headstalls with a buckle on the top. Although I agree that this is a presentation (in-hand) halter, I wouldn't say that it is specifically for donkies. Take the curb off and add a chain lead and you have a plain in hand halter for something like a Morgan. Generally, show halters or bridles (more commonly in arabian, saddlebred, miniature and morgan specific tack) will have a buckle at the poll in order to create a more refined and less 'busy' appearance to the item.
The arabian world loves this idea. Arab show halters tend toward the buckle at the poll.
Though it may not be "made" to be a hack, it seems like it would operate in a similar manner as tying reins to your halter. If you want to use it like that, there really isn't any reason you can't.
Proud owner of ~Mana: 6yo Arabian gelding~Pearl 13yo Arabian~Danzer 14yo Arabian mare~ Tiny mini filly