Hello- After reading of the damge done by the splinters thrown by the shells fired at the C.S.S. Atlanta by the monitor U.S.S. Weehawken's 11 and 15 inch Dahlgrens I should not wonder that someone addressed a safety concern.
David, was the reference you'd read concerning internal protection against spalling post June, 1863?
Putting myself in the times and considering the technology available I'd consider canvas, in layers, as a protective material against the slings and arrows of outrageous fortune.
I'd never read of the internal Oak courses of a casemate ironclad being run diagonally. Reference I'd read stated the inner course being horozontal and the outer course run vetrical. Diagonal installation does not read as a bad idea. Any displacement of the case would be spread over a greater area.