That's an "efficient" looking design and likely really great for loading BP revolvers. But those revolvers have steel backstraps as well as the TG which extends all the way to the backstrap. That's a solid setup.
But for flintlock pistols that don't have that steel or brass reinforcement, a leather padded curved insert that would allow the curve of the pistol grip would be far better in spreading out the downward loading forces across the pistol grip.
A hinge like the one in that photo is a good idea and perhaps could be incorporated into what I'm wanting to make. Also, extending the bottom piece so the unit could be "C" Clamped onto a shooting bench would be a "must have" upgrade.
But thanks for the post as the hinge idea to make the unit more compact is a really good idea.
Edit: I also think the height of the unit for loading a flintlock or cap lock pistol would need to be high enough so the user could wrap his\her fingers around the barrel and the top of the unit at the same time, in order to "hold" the pistol tight against the padded top. If the unit is to low, it may negate any advantage of keeping the pistol snug against the leather top....and the leather padded curve.
