the chance they will drop is very, very, very, very .....small (I have been doing the quests/wanteds since they came out and have yet to get one---have gotten a few of the green weapons, like 3 or 4, but no minis....this is why they are priced so high).
link for discussion about the drop rate and such: https://wiki.guildwars.com/wiki/Talk:Mini…onfessor_Dorian
I had no idea that minis dropped in WiK. I always thought you got all of them from Royal Gifts but just looking now, I see that two of them (Dorian and Isaiah) are from drops. But who or what drops them?
My understanding is that all White Mantle and Peacekeeper foes from WiK are capable of dropping them, it's just that the drop rate is miniscule. I've never had one drop myself either.
there is also a peacekeeper (drops from you guessed it,---peacekeepers). If you check out the wiki it says who they all drop from (the link I gave you can backtrack from ...)
Yeah, I have never gotten any mini to drop from the mobs---have gotten 3 from gifts as well as about ~4 greens from the mobs...and I used to do a bunch of the quests 2-3x every month (when I had all 3 of the bounties) with a bunch of my characters (would do Vakar, Sarnia and Lev since you could hit them all in one area). Tried doing it with as few heroes as possible to make sure if anything dropped it would be mine and not a hero's! And have nothing to show for it execpt tons of warsupplies, confessor orders and such.
The problem is that it's only speculation and the wiki page says the guy who had a drop had it while WiK main quest was active. So, as far as I understand it, there's no proof of an actual mini drop outside WiK main quest-line. Although I know the drop rate is very low.
I sldo went to Kamadan, to ask WiK mini sellers where they got the drop from but unfortunately they never answered.
Well, at least we still have an unsolved mystery in this game after all these years. 
https://wiki.guildwars.com/wiki/Talk:Mini…keeper_Enforcer
Atleast there is claim getting peackeeper from wanted quest
http://s150.photobucket.com/user/guildwars…01Mini.jpg.html
Sadly the 2nd imae doesnt say where it dropped 
So yeah not actual proof but the changes are just superb low and small so dare say noneone whod even try get it for 5years wouldnt get1
I have asked around and no one I have asked has replied to this...so I am going to bump this topic and see if anyone has gotten one since this was posted...or has the answer.
