The biggest problem with the .500 Smith or for that matter, the 50 AE, is that of sectional density. 300 gr.s of bullet sounds like a lot, but when it's got a cross section of.500" it's darn stubby, not far from a round ball. The 45--70 (or 90, 110, etc.) gets it's great reputation as a killer of big game from the sectional density, which with bullets of equal constructin always equates to better penetration.
As for rechambering .500 Handi's to any rifle cartridge, forget it. They all have bores of .510 or larger, with the exception of a couple long obsolete nitro for black rounds like the 577/500 Express, which has a .500 nomial bullet diameter. If a fella has a Smith in .500, why heck he should own a Handi in the same calibre. Can't see a real demand for the cartridge in and of itself. Ammo is really high for what it is, not formable from another case, jacketed bullets are plenty stiff in price, and there aren't a lot of heavy bullet moulds for it yet.
Me? I'm still holding out for a 405 Winney barrel. Sectional density and velocity all in one neat package.