Can a custom be made?
Well for the right amount of money, anything is possible. But looking at the specs of the .500 S&W, it looks very possible. The diameter at the base of the 500 is .526", while the base diameter of a standard magnum is .532. Just 6/1000th of an inch difference. I'd say go for it. The difference in length would make some problems. But the easy way around that is to make it a single shot. Or another possibility would be to load .50BMG bullets into the 500 S&W cases, to an OAL of 3.34".
I have a Spanish Mauser, with a 30" .45 cal. muzzleloading barrel, made to shoot the .45 ACP. And pretty darn accurately. So to answer that part of the question, anything is possible. If you can dream it up, you'll be able to find someone that is willing to take your money and build it.
As far as factory guns, not that I've seen, yet. I'd say look for them in this order; break action single shot (someone probably already makes a barrel for the Encore), falling block single shot, then a lever action. The last two could be reversed, it just depends on if Marlin or Ruger jump on it first.
Wyman