yoopper; I was thinking along similar lines as you describe; I face the same problem as the OP; the only place i can shoot i cant shoot lead; and the only lead-free pellet I can find goes only up to .25cal.
As others say, the properties of lead are hard to beat; especially its workability and softness. Many other materials might chafe more in the barrel or damage your rifling.
But why restrict yourself to a single material? Take a piece if threaded metal rod, slightly lower OD than your bore, cut to length, and shrink a piece of PTFE shrink wrap over it? That should give a compliant layer to engage your rifling, cant get lower friction than PTFE, and the threads should hopefully suffice to keep the PTFE in place.
It would require some tinkering to find thread (or rod) diameters in combination with the right shrink tube to get a good fit; but PTFE sitting in a dimpled surface should have quite some give. Copper or steel rods would be obvious choices, but if you can get your hands on a custom mold, you could also cast in zinc, and get it exactly to size including the shrink tube.
This would, (should it work well in practice), be like a regular teflon coated bullet. Potential higher muzzle velocities, and great penetration, though no expansion. But as a variation, your core could also be steel powder fused with some wax, wax-shotgun-shell style, for awesome energy dumping (is this generally accepted practice or a taofledermaus thing? either way a great resource for experimental ammo). Probably still want to do the teflon coating then as steel powder would not be kind to your barrel.
Alternatively, some hard waxes could also be cast around some metal core in an existing bullet mold, along the lines you describe.
Doesnt sound that more labor intensive than casting your own lead; although getting it set up probably requires a lot of tinkering. I dont currently own any big bores myself to test this on but im eager to start tinkering with my budget 25cal.
If anyone has experience or insight along these lines, please let me know!