Just an idea i had that works too. I made an Invasion Map with a reverse CTF Phase. So you have to bring a Core to a Generator to override a system and unlock the One Way Shields(That dissapear after that Phase) to get to the next Phase. Just some cool thing i wanted to share. I came up with this idea when i wanted an Elite Assault phase but it spawned those horrible human bombs. It spawns the human Data Core, but it could be covie tech too. For Spartans, it sucks to carry a purple thing, so i would just recommend a Bomb. Meh, just an idea that will hopefully inspie people for Covenant Assault Phases.
Technically, the Data Core isn't Human. At least, not according to Bungie. Notice how on Spire the core is called a Power Core, and on Boneyard it's called a Navigation Core. It has aesthetics that could match with Human, Covenent and Forerunner designs, so it can really be used however you want it. Just felt like sharing. Oh, and reverse CTF isn't anything new : ) Why did you just repeat exactly what the OP said? My guess is you thought he was asking how he could accomplish a reverse CTF game, and not telling us how to do one.
Just have the Elites be attackers, Spartans be defenders, and place the Spartan's Core in the Elite Base. The only problem with this is that there's no arming time...