It is possible, I've seen it in game.I don't think you can reforge below -20 MW. I wasn't trying to make bokutos on TC, but rather Exquisite metal weapons with 100% elemental weapon damages. I made many with MW and they never went below -20.
I think the only way to get -15 MW is to get it via loot. It seems like they used the normal imbuing max intensities for reforging. I hope you prove me wrong, but I didn't get what you are looking for with a hundred equisite reforge attempts.
Bummer. But I understand.Ah sorry. I'm a perfectionist, I wanted to enhance with blood wood to get the hpr.