Fascinating project! I'm considering roughly the same idea.
You mentioned you can fire modified .410s--what needs to happen to them? Will virgin cartridges just fireform to the chamber? Any problem with the 3" hulls in the ~2.90" chamber? Or do you have to trim them?
I imagine you'd need to keep the pressures to .410 levels in the plastic cases (ANSI lists 13,500 PSIg for 3" and 12,500 PSIg for 2.5").
I would imagine all brass hulls would be used. Other wise I am sure the 444M would out perform it.
Back to the 444M being the weak little sister, I do not think so. If you load up the case to it's full potential, it will run at a higher pressure than you can run the 45-70 at, due to a stronger case. I know that some have loaded the 45-70 to 50K psi. The 444M is based on the 30-06 which runs at 60K psi. So if you load up the 444M to it's maximum pressure, not the anaemic industry standard of 44K CUP, you will out preform the 45-70 with similar weight bullets. If both are loaded up to it's maximum pressure of 50K psi and 60psi respectfully. Now some will argue that it is unsafe to load the 444M above industry standard. If you use that argument with the 444M, then you should apply the same standard to the 45-70 and hold it's pressure to a standard of 28K CUP.
Now there are a couple of problems with loading the 444M up to 60K psi. You have to have an action strong enough to handle 60K psi. Also, the bullet selection for the 45-70 is much better than for the 444M. I am not sure if there is a bullet that would give satisfactory performance at 60K psi. But if there is, it would truly be a real power house, not a weak little sister.
One other point - Hornady makes LEVERevolution ammo. The 444M shoots a 265 grain bullet at 2325 fps. The 45-70 shoots a 250 grain bullet at 2025 fps. Both out of a 24" test barrel. I think they load both up to safe levels for a lever action. Which has more punch?
Of course the 444M does not have the history that the 45-70 has. I am not sure how much this comes into play in the woods. I am sure it is not as much as at the range. I am not putting down the 45-70, it has been around for a long, long time and is still very popular for good reason. I just do not think that the 444M is weaker, I think if it is in a strong action with well constructed bullets, it can be the equal of the 45-70, if not better.
Good Shooting and Good Luck