Three things here .
1st , Is that I don't think that you could get enough crimp to hold the shot capsule , with the 45/70 being a .458" and the caps being around .450"
2nd , The 45/70 having a slight taper , the cap most likely would just drop all the way into the case .
3rd , The amount of shot that you could get in that tiny cap would not even come close to that of a 2 1/2" 410 shell , so there is no real advantage .
Just my thinking on this .
stimpy