The local sporting goods store has 200 grain copper-plated .429 bullets for sale in bulk. They are lead but have no lubrication groove (probably meant for 44 Special pistol reloads). However, they are the right weight and diameter to also be loaded in the 44-40 cartridge. Without lube, should reloading in 44-40 be limited to smokeless powders? What would likely happen if they were loaded in Black Powder 44-40 reloads?

No. Many period 44-40 bullets were crimped over the bullets ogive, back in the day. I'm not certain if the copper plating would be any sort of issue with BP.Hopefully the bullet nose is short enough to fit in your cylinder, if these are actually 44 mag/SPL pattern bullets. 44-40 bullets had a shorter nose. IIRC, BP requires a slightly compressed load. This could be an issue if you're using the wrong bullet design. I think I'd spend a little more time looking for the correct bullets if it were my project.