I can't believe that nobody has just taken the easy route and said, "Light load for .460 S&W?! Yeah, it's called
.45 Colt! " I mean, c'mon, ya got to see that one coming.
If using Unique, I'd suggest you send an e-mail to Ben Amonette at ATK. He's the guy that answers technical assistance questions regarding Alliant powders and he may have some suggestions.
I don't think he'll endorse you attempting to use pulldown powder from .50 BMG rounds. I don't think it will work worth a darn...but that powder must be outrageously slow burning and I don't believe it would be possible to build an over-pressure round with that powder in .460 S&W.
So if you attempt it, your biggest risk is going to be with a stuck bullet. Definitely use a lead bullet as that'll be easier to whack back down the bore if it doesn't end well. And ensure that every trigger pull gives you a hole in paper - if you fire one behind another that's stuck, you'll likely wreck that revolver.