I don't own any polygonal barrels, but from what I've read, the bullets often don't deform perfectly to fill the rifling corners of them, allowing gas bypass leakage at those places. That causes gas cutting and splatters lead in the bore pretty badly. Moly coating and plastic coating won't likely help with that as the same gas cutting would still occur. I think a heavy plated bullet, like Berry's will probably have the best chance of surviving in them if you don't want to stick to jacketed bullets.
The only other thing it occurs to me to try would be applying a permanent lube to the bore itself, so leading doesn't adhere as well. There are a couple out there. The easiest to apply is Sprinco Plate+ Silver. You just submerge the barrel in it for three days. People worry about breech thrust increasing if the stuff gets into the chamber. I haven't seen a problem with that in .45 ACP, where the case normally backs up instead of stretching anyway, but it runs at lower pressure than 9's and 40's. You can mask your chamber by painting the inside with nail polish or layout die or even Magic Marker. Plate+ is reusable, so pull the barrel out, drain the leftover back into the bottle. Wipe the excess off the barrel and patch it out of the bore. If you masked the chamber, then use acetone or other appropriate solvent to clean the chamber out. Solvents don't remove this stuff. The maker says it's good for about 1000 rounds, but if you patch some through the bore after cleaning, you keep the bore replenished. Again, wipe excess from the chamber after doing that if you don't want it to attach there.
The other product is Shooter's Solutions Moly Fusion. This is mixed with mineral oil and patched into the bore. Again, if you don't want it in the chamber, mask that area. This stuff takes about 20 minutes to apply. You pre-heat the barrel to around 150°F and put it on a bore mop and keep running it back and forth in contact with the metal, periodically re-warming with a hair dryer or hot air gun as needed. It will discolor plain steel to a grayish appearance. It makes the metal surface sort of waxy feeling, but this stuff is 100% dry. It also needs occasional replenishing. There are photos on the product's web site of the result of treating the bore of a magnum rifle with it and half copper fouling stopped completely by its presence.