If the 45-70 is illegal for dangerous game in RSA, that is news to me, though I honestly don't doubt your word. My RSA PH (Jeff Cooper's also) carries a 45-70 as his hunter backup gun. I know they're not illegal for DG in Tanzania, because the Government Game Scout was more than a little impressed by the THREE Cape Buff I killed with a 45-70 in his presence during MY first DG hunt.
I think you may be confusing classic 45-70 loadings with what is available today. The Buffalo Bore 430 grain LFN at 1925 FPS pretty much makes it into the the Heavy Rifle Category. Randy Garrett's 420 Grain Hammerheads have been known to pass lengthwise thru a Buff and I personally placed 7 hits broadside thru one, all of which exited. Hardly what I'd call "stunt hunting".
Still, if the 45-70 is illegal for Dangerous Game in South Africa, I'd like to know about it. Where is that information provided?