Dennis Potter,
As you have said you have found pictures of added on pistol grip to single shots and I think most of what has been shown here on the post, as well done as they are, may be similar to what you have already reviewed (just a guess on my part). But I think that a very nice grip could be fashioned to straddle the straight grip and then inletted onto the grip. You would remove wood to a small depth all the way around the rifle grip and then carve the new wood grip to fit over the area where the wood was removed. It would look similar to the rifle in the bsteele link.
Here is a link to an upcoming auction of a Maynard with a pistol grip. Look pass the form of the grip itself but look how the lines are around the edge. Although this is not a fitted piece of wood, it could be. Let you imagination flow and hopefully you can see it.
Maynards If you want to discuss you can pm me and we can connect
