CitizenHats

This is a Citizens2 addon! You can get it here: https://www.spigotmc.org/resources/citizens.13811/

CitizenHats v1.35

This plugin allows players to put any type of block on their NPCs as a hat! It is as simple as it gets!
Another common issue with these types of plugins has been solved with my version -- NPC hats removing themselves after a server reload/restart.

NPCs are now loaded along with chunks! No need to use /loadnpcs anymore, however, the option is there in case of buggery.

NPC Hat

Invisible NPC

Commands
/npchat - Adds the item in your hand to the head of a selected NPC
/loadnpcs - Loads the saved NPC hats & Invisibility onto NPCs after a server reload.
/npc invisible (/npc i, /npc invis) sets the selected NPC to invisible.

Permissions
citizenhats.npchat - Allows the player to add a hat to any NPC
citizenhats.loadnpcs - Allows the player to load all NPC hats onto NPCs.
citizenhats.loadnpcs.invisible - Allows the player to load all NPC hats and Invisible NPCs.
citizenhats.npc.invisible - Allows the player to set a selected NPC as Invisible.

Please feel free to let me know of any issues or suggestions for future updates.