DP, you might take a look at the buttplate that Track lists as BP-HAWK-E-B, this has enough curve to look right on a halfstock, but not so much that it looks uncomfortable. When I was choosing the plate for my current build I made plywood cutouts of different plates to try on my shoulder, but still bought three different plates before I was comfortable.
You have only mentioned halfstock percussions, but a style that is dear to me is the fullstock percussion from the transition time. I used to see these at shoots, with a generic/late Lancaster style stock. These were often very plain without a patchbox or capbox, nor any carving or inlays other than a hunter's star or cresent moon in the cheek piece. They were intended to shoot and always seemed to belong to a serious shooter.
I like plain rifles that have to stand on their architechture and accuracy, and these take well to a simple apperture sight on the tang, such as the "thumbscrew' sight. While talking about apperture sights, I want to complain that there is such a lack of simple clean apperture sights available that are appropriate to long rifles. I make some of my sights myself, but my "mill" is a small drill press with an inexpensive cross slide, so its something of a struggle.
I think your project is really interesting.
Greg Newcomb
