Vernier adjustable peep sites were used extensively during international competitions in the mid to late 1800's.
Although the sites were often fitted to the tang or wrist of the stock they were also often fitted to the heel of the stock to lengthen the site line to it's practical maximum, this was possible because many of the top shooters of the time shot from the "Back" position where they would lay on their back and rest the barrel of the gun on either their knees or even the toes of their boots. In this position the left hand can either be used to steady the gun just forward of the breach or it can come across the body and support the heel of the stock.
I have been using this postion myself and have found that looping the sling around the toe of my boot helps in controling the the recoil. It's a very effective stance and quite comfortable as long as you don't hold your head up in the aiming position for too long at a time.
Mac Attack, the front sight normally looks fuzzy if the hole in the peep site is too large, or possibly your eye is a little too close to it. With modern target sights the rear site is normally adjustable and you reduce it until you get a clearly focused fore site. It works in exactly the same way as increasing your focal length by reducing the aperture on a camera.