You might have to wait until one happens to check in. There’s a limited number of us who check the forum daily, but there’s quite a few who stop by every now and then. I really expect Clay Allison to chime in on this one. I’m sure he’s done it.
Not being a bow hunter myself and assuming you’ve already failed at clamping in the base vise, I would think that you’d be able to clamp most fixed, two-bladed heads in the Low Angle Adapter – maybe with the point of the blade extended off to the forward side, depending on the shape of the blade. You’d only need to clamp at the very back of the lower edge. This would expose all or nearly all of the upper blade for sharpening.
Edit: Understand that the Low Angle Adapter as supplied (unless they’ve since modified the design), will not fit in a Gen 3 vise, which is a little wider.