I also use 405gr bullets from MBC and they work just fine in my original TrapDoor. With the low pressures generated by TrapDoor loads I see no need for the coated bullets. I get no leading at all with the bullets from the MBC which are in fact .459".
Just as a side note, there are a hand full of very good powders for the 45-70 at lower pressures. They include Unique, 2400, Trail Boss, 4198, RL-7 and a few more. I have been using AA5744 for the past two years and I have no plans on changing. It produces very accurate and clean burning 45-70 ammo for me. It works very well with both 300gr and 405gr Cast bullets.