Well, let's see...
The grill comes fairly well below the chin, but leaving the maille unattached could possibly leave an opening for a freak shot if the wearer's head was leaning far back, so I'm really just thinking maximum safety here. And as one of you mentioned, this arrangement is about as safe as you can get for the throat. Because the grill does come down reasonably far, it wouldn't look quite right to pull the top edge of the aventail up under it to emulate the period wearing. It does have the triangles, BTW-that was one reason why my client (Hi, Chris!) decided to go with one of Master Knuut's long aventails. It is a very excellent aventail, BTW-thin flat stainless wire that is welded. Looks like riveted at first glance and the weight is quite reasonable, unlike so many other SCA aventails.
As to the authenticity part-since the helm has a fixed bargrill, it's not exactly accurate anyway... (And yes, I know about the ONE iconographic reference to a barred bascinet

, but this is a different arrangement.) It's intended to be a representation of an open-faced bascinet. The main concerns I have here and now are safety, comfort, and practicality. If it looked as right as possible, that wouldn't hurt either...
I'll try to get a pic posted tomorrow.