They could do this using player made npcs, (invisible npcs with the tools (mobs) on their heads) but it would probably need to be coded in. It would be a invisible npc with the tool on its head, player summoned, and it would probably need to be set up with a path by the player for it to walk. They'd have to do it so it summons the npc where the player is standing, similar to the elf, where it can be pushed around. I could see that being a lot of work for the devs. It is possible but I would assume it require permissions to a plugin to be able to set up npcs like the "mobs" Or, they can let players add them automatically with a plugin that places armor stands with the tool (mob) on it. Its a great idea if they could figure it out.
I think with the kind of plugin they use currently to summon mobs though that they would eventually be killed off when the housing server restarts. I think its a zombie pigman made invisible with the tool on its head, used as their mobs or similar. I don't think its possible with the type of current plugin they have to make npc mobs especially custom ones by players.