Barrel leading I can prevent or deal with easily enough. It's keeping it out of the gas system that I'm hoping to avoid. I haven't tried powder coating at all and wonder if anyone else might have tried it - at risk of great cleaning troubles- in a gas rifle.
Your concerns are well founded, and yes, powder coating can solve that problem.
I don't shoot a Mini-30, but do shoot the same cast bullets you are probably considering in a couple different AR15s in 300 Blk, 308, and a 30 Herrett wildcat. The lube-groove Lee 155gr gas checked bullet is a good one for this, as is the very similar NOE version.
You will want to use a gas check with these, even powder coated, and hardness does still matter for accuracy. For full power loads I get best results water quenching my wheel weight alloy; I do this by pouring water on the bullets when they come out of the coating oven. (Quenching out of the mold won't do much if you anneal them again in the baking oven.)