Not a BMG, but a much small case head with the 300WSM which had too much breech thrust even at 150gr 30-06 velocities, when 3000fps was approached, the action flexed creating a gap at the top of the barrel and breech face, I think if the frame was changed to provide support closer to the latch or they used a larger diameter pin and a more robust latch, belted magnums would be a possibility, but as it is, anything much bigger that what's factory chambered now isn't possible.
Tim