Never seen or heard of one either... largest I've seen is a .45/70 down at Whitehorse Arms...

I'd be much more concerned about the Martini Henry's action strength over whether a 450-400 round would fit... despite its impressive size, the action is after all made with ~100 year old steels (and emerging knowledge of materials science and heat treating) for the low pressure .577/450 black powder cartridge

A 450-400 Jeffey based on a Ruger No 1 would leave a much greater safety margin IMO and be able to get the most out of the cartridge...