I suspect that 500 S&W, 460 S&W and 454 Casull may cause pressure issues in a 'howdah' style action. However 480 Ruger may be suitable. Not as hot as 475 Linebaugh but quite adequate.

Another cartridge which shouldn't be ignored is .45 Colt loaded to the Ruger/Freedom Arms level. Not being greater than 45 cal, licensing would be easier for those of us in the Peoples Republic of Oz.