"I've gotta have some kind of peephole or notch to look through."
Why? The bead alone is sufficient for deer hunting out to at least 50 yards and after you become more experienced and more confident with it you will be taking shots up to 75 yards with confidence. That said, if you want a rear sight a new higher front may have to he added, but all in all, other than possibly weakening the action near the breech face that takes the thrust and pressure of the firing of the shell your idea seems very interesting. No windage adjustment but elevation adjustment would be fairly simple by turning the screw in or out. I would suggest a fine thread screw. You might even place a small washer or circle of some kind in the slot and make a peep sight. This sounds better all the time. Great thought, lets here more ideas please from those who may be more informed on the strength of the action and what may happen by drilling and tapping a small hole at that location, would could even place a small nut on the screw to use as a locking nut....<><....
P.S. - Welcome aboard and Merry Christmas to you preventec47, I hope to see many more of your posts and ideas here....<><....