It's just a simple magenta effect that spins around the unit.
I came up with a name Empathic Bond, since I'm using it with an ability based on Spirit Link. It should be attached to the unit's origin and it can be used as any kind of buff, effect and maybe even aura.
I am a massive fan of Beta stuff brought (back) into Warcraft, so you already have my bowaxesword vote. More than that, it's a really simple, pretty & useful effect, so I'm even more on board.
My only hang-up is the use of custom texture; I know what you said, but I'd be surprised if there wasn't something just as good in the MPQs; the tail end of the Graveyard ghost? Textures\firering6.blp? A magenta-ized Tornado.blp? Hmm.