Daryl,
I was doing exactly what you described in your post “that it shows a sectioned .45/70 case, with a very small charge of fast burning powder in the bottom of the case, held there with a tiny tuft of Dacron.” The load I was using was about 10 gr of Unique with a Ľ sheet of single ply tissue holding the powder over the primer. I never had a problem but I was only using a 300 gr bullet. Where the problem of ringed chambers became most evident was with the long range shootsen shooters who seated the bullet into the rifling then loaded a charge of powder in the case. They often used a fast powder with a card wad to hold the powder in place. Charlie Dell did a bunch of experimenting to determine the cause of the chamber ringing. He found he could ring a chamber with out using a over powder wad.
I noticed you were using cardboard wads in your BP loads. Just be sure to add the weight of the wads to that of the bullet when deciding on what charge to use. You have it right in you MUST fill the case and compress the powder.
Bill