I'm sure this has been posted before, but here it goes in case I'm wrong.
Just use your ghoul's Leap skill on any npc and a guard will spawn (as if attacked by a hostile). Does not matter if the npc is friendly or neutral, altough if used on a neutral npc, the npc will attack your ghoul and eventually you too. I've tested it in places where you can find guards, like capital cities or quest hubs. Note that not all towns have guards, like Agmar's Hammer in Dragonblight. Try to attack a non-guard npc, as I've never gotten a guard spawn by "attacking" a guard.
Does not work in sanctuaries, like Dalaran.
This will give you no uber powers, but if you want to fill a town with additional guards, go ahead.
Be careful though, some guards will attack you even if they are friendly. For example the Moa'ki Warriors in Moa'ki Harbor will attack you even if you are friendly with them.
It's possible the npc has to be pvp flagged to get a guard to spawn
For extra fun, make your ghoul leap to a flight master6):