If you go to a hero's unit details screen and look at abilities, one of those should tell you what items that particular hero can equip.
Shit, didn't even think to look there. Will give that a try right now....
2 out of the three didn't say anything about weapon proficiencies.
Korthox the Ice Lord: Sword Mastery
Silindros the Avenger: nothing listed
Ifris the Engulfed: nothing listed
Checked my regular (non-minor) heroes as well
Varda: Sword Mastery
Drogon: Axe Mastery
Naphaz the Warlord: Staff Mastery
So it appears, couple heroes just need to be updated with info.
Guess I'd still ask to have something changed around, as this is not the most obvious spot to be looking for that info. Or if leaving it that way, maybe have the weapons state to look for that specific 'mastery' in the details section of the heroes, so that newcomers know where to look for it at least. i.e. have either the recipe or the actual item have something in its description saying "Equipable by Heroes with the 'X' Mastery ability."
Either way, thanks for the heads-up Heavenfall!