I have checked Gadalion, Isslana, Cazel (thought someone might comment to charm a merch here - nope... though I did find a comment or two echoing something I believed in classic - disease dots should block mob regen
), Synthan, Elesseryl Terussar, Doran Vargnus, Tisiana … None of them had an mention of merchant charming as a strategy. You'd think someone would have mentioned a merchant being a decent charm pet somewhere.
Like I said, if anyone can give a list of merchants that are obvious charm pets I'll sift through comments trying to find definitive evidence that merchants either could or could not be charmed.