Slightly off-topic but I quickly wanted to respond to this anyway. I did a ton of testing on the 5.4 PTR regarding trinkets and Haromm's used to proc from everything besides pet and guardian attacks. This includes Murder of Crows, cloaks, Explosive Trap, Glaive Toss, meta gems, engineering bombs and other things I can't even remember anymore. Since I've only just gotten Haromms on live* I haven't properly tested everything yet but I don't think anything has changed. Although small sidenote, Piercing Shot does not proc Haromms for as far as I could tell. Haven't done much proper testing yet on Marksmanship but after 200 ticks it still hasn't procced. Small sample size but... come on it has 18% chance to trigger.
*Absolute unbelievable, been clearing since the raid opened and we've seen like 2 of them so far on 25 man. I had to coin mine