Can you make a NPC wear a guild tag?
 

Anyone know if this is possible?

Client wouldn't display it if you could, unless you modify the code to make that NPC show up as a player entity to other players.

you could do a work around and use NPC second name - the one usually shown in (), and put a guild name there.
But again some server code changes will needed to write a new quest command to alter sub name in real time and write it into DB