In essense these great people only have the hero promo (+1xp slowly up to 100xp like all the heroes). Furthermore, the only other thing they can do is transform into a "normal buildable" (axeman, spearman, chariot, archer, horseman, adept, etc. in FFH) by paying an upgrade fee like you would pay if you upgrade for example a warrior to an axeman. The added advantage is off course that it has the hero promotion (unlocking specific promo's and the +1xp without combat). This civ will than have no hero unit but I would keep the captain units (depending on if we give them names I think also). The king unit is also still there.
Something else I came across, in FFH2 there is a sort of king unit in the game. For the mercurians and infernals, there is a unit of the leader which you get when starting and also when that one dies, you loose leadertraits (or parts of them or something). It might actually be pretty cool to use a similar thing (although I don't like losing all traits)