The proprietary part comes when buying the brass and ammunition. The same way Weatherby was for a short while, until Nosler started making their brass and ammunition. Lazzeroni is the only ones that make it to those dimensions ready to be loaded, that is unless you want to form it from something else. As said before, even though the Lazzeroni line is said to not have any parent case, they seem to be somewhat 416 rigby based, atleast the long magnums do. Dies will also be ridiculously priced from wherever you get them, just because its an obscure cartridge. But as far as a smith chambering to that cartridge, I don't see where you couldn't do what you want.
On another note, if you have a wild hair for a .284 caliber round, there is the 7mm Remington Ultra Mag. You could probably duplicate the Warbird with the right powder and barrel length and it would be more economical.
But on the other hand, if you're just wanting something nobody else has, I can appreciate your enthusiasm, as I'm the same way. I want a 375/416 improved, but everyone tells me go with a 378 Weatherby or 375 Snipe-Tac. Thing is, I don't wanna..