Yep I have even heard of the Pedersoli .45-70 being converted to .450 No. 2 NE. I think it should be relatively simple to deepen the chambers on a .45-70 double to accept .45-90 or .45-120, maybe depending on the taper of the barrels. And, handloading to pressure levels acceptable in the Baikal, I would think you could get a significant performance increase with the larger case capacity. I'm sure a gunsmith familiar with Baikals, doubles and break action guns in general could tell you a lot more. I believe Daryl S. who frequents this forum is fairly expert on loading these particular cartridges to maximum potential, given the pressure limitations.

One thing to keep in mind is that those Baikal doubles weigh in at only around 7 pounds. It won't take too much hotrodding to make them light for cailiber.