Drew - .457 down to .451 isn't much of a problem with PURE lead but any hardening alloys will turn the effort into a nightmare (sounds like you already know). Why not just shoot full bore conicals? Big, heavy conicals don't mix well with sabots in my experience.
The only exception being some White Powerstars for me in my .45s. They are also available sized for .50 cals.
http://www.whiterifles.com/store/mor...?Product_ID=65 I'm told Buffalo Bullets makes some nicely performing bigger sabot combos but have no experience with them - then I've drifted into the ready-made world...
Dan at MountainMolds.com can fix you up with most any mould you desire for not much more than off-the-shelf pricing. You can even design your own with his software. I've heard nothing but accolades for his moulds.